Maybe it starts in Diagon Alley, when Hermione and Ron are shopping for their Third Year supplies, and they catch sight of Harry and call him over to join them, and he has new clothes and glasses, and an enormous black dog accompanying him, and he just waves away all Hermione's entirely reasonable concerns about him frittering his money away on unnecessary things. All right, his old clothes were a bit scruffy, and she can see the point of getting new shoes, but day robes? Surely a couple of new pairs of jeans and a few t-shirts would have been quite enough. Plenty of people wear normal clothes in the Alley, but it's not like you can wear wizarding robes in the muggle world. And Ron always gets so twitchy when reminded other people have more money than him; she'd have expected Harry to show more consideration.
Maybe it starts when Harry announces he's dropping Divination in favour of Runes and Arithmancy, leaving Ron on his own, and he'll have fewer free periods to hang out with him, too, and it doesn't matter how upset Ron gets, how disappointed and frustrated, Harry refuses to go back to the original plan, the one they decided together. This isn't like him at all. And he has the nerve to suggest that if Ron cares that much, maybe Ron should be the one to take Runes and Arithmancy, which is just mental. Everyone knows those are the really hard subjects. Percy took those, and Bill. And on top of that, there's Hermione getting that stupid cat, and Harry's dog going mental, and really, Ron isn't having a good day at all. He thinks everyone should cut him a bit more slack, and also leave Scabbers be, for Godric's sake.
Maybe it starts when they've been back at Hogwarts a month, and the tenor of Harry's interactions with the other students has shifted. It was bad enough Dean Thomas suggested a pick-up football game within Harry's hearing, but for Harry to go, and then make it a regular habit! Hermione thinks running around and wallowing in mud is unbelievably childish, and Ron doesn't see the point of a sport played without broomsticks and only one ball. And now he's on first-name terms with Justin Finch-Fletchley and who knows who else, and Hermione can't swear to it, but she's almost sure she caught sight of him talking to Malfoy once, even shaking his hand, and certainly he exchanges nods and civil-albeit-curt greetings with him when he sees them, and he seems to be participating in the same exchanges of gestures she's seen the Slytherins do with each other (and some of the Ravenclaws and a few of the stiffer Gryffindors like Percy), the ones Ron refused to explain to her on the grounds that they were 'only for blood purists and pompous gits.' Maybe it starts when Hermione and Ron get asked to Dumbledore's office, and he seems so very concerned about the changes in Harry, so sad and disappointed Hermione and Ron haven't done a better job of keeping him away from bad influences. Though neither of them really has a clue where all these changes are coming from in the first place. Most of the one-on-one conversations Harry seems to have with Gryffindors who aren't them are with Neville, and it's hard to see Neville as any kind of corrupting figure. He's just so... Neville. Still, Dumbledore told them they were doing the right thing, bringing their concerns about Harry to him, and that was such a relief to hear.
Maybe it starts when they begin to act on some of the ideas that came to them after that meeting. (Looking back, it was hard to tell how many of them had been direct suggestions from the Headmaster, and how many of them had just... arisen in their minds, following on from the conversation they'd had.) Ron shares a dorm with Harry; it should have been easy enough to get into his trunk, get a feel for quite how bad his shopping spree had been, and whether any of his new books were in the same league as that diary of Ginny's. (He won't let what had happened to his sister happen to Harry, not again, he doesn't think he can bear to re-live that.) But not only does the lock not let him in, it singes his fingers, and the look on Harry's face when he sees the tube of burn cream on Ron's bedside table hurts worse than his hands do. Hermione thinks the best way of finding out what dangerous ideas Harry has been getting is to proofread his essays for him, see if he mentions any books or dubious concepts by name, and it's a bonus that she gets to actually do something nice for her friend. Only he says he appreciates her good intentions, but he never actually asked her to do that, and it's great that she caught a couple of spelling mistakes for him, but otherwise he's just going to write the essay out again as he had it before. He doesn't need that extra paragraph on the tense politics of Henry VI's regency when the essay was supposed to be about the use historical animagi made of their ability to transform; he'd rather focus on the whole thing with Lisette de Lapin using her small Animagus form to escape from prison, and keep the political stuff as a brief aside (though he does end up adding another sentence about how some historians found the timing of Lisette's flight, relative to Charles VI's death, rather suspicious, and that a small and unremarkable animal form might also be useful for an assassin). Ron, hesitantly, tries talking to Dean about Harry, tries talking to Harry about Dean, but neither of them really says anything nasty about the other that he could then pass on. It was much easier when Harry genuinely was overwhelmed, and Ron could just tell people to back off knowing that Harry actually wanted that. Ron makes remarks about swotty Ravens and slimy Snakes and useless Puffs, but that just gets Harry and Hermione and Neville as well all giving him disappointed looks. It's better when he mentions last year and how badly some people behaved when they thought Harry might be the heir, and Harry at least agrees it wasn't right or nice, but then he goes and says that maybe if people knew him better, they'd be less likely to think bad things about him, and Ron just... doesn't have a comeback for that. Dumbledore would. So he just tells Harry to be careful, and how he doesn't want to see him hurt.
Maybe it starts with the obvious downside of Hermione trying to guard Harry from dubious ideology: she has to learn about it herself in order to do that. She won't know which of the books he might be reading are unsuitable if she doesn't at least have a list of titles, and often she ends up resorting to reading things she knows Harry mustn't even get a glimpse of. The Headmaster's very emphatic about the importance of discretion, and at first she's proud to be trusted, proud to be considered mature enough to handle it, but it's tiring and lonely and she has so much homework already, and then there's all the Buckbeak research, and she thought the books were from Dumbledore's private collection, at first, but she learns enough about heraldry in order to be watchful for Harry getting drawn into 'undue pride in one's ancestry, when it's the person that matters', that she realises that quite a few of those books had Potter and Peverell bookplates, or their emblems stamped into the leather bindings. It feels odd hiding those from Harry, when she knows that part of why he treasures his cloak and photo album so fiercely is that he has so very little of his parents, of his wider family. Of course she's just helping to keep him safe, and that's more important than ever with Sirius Black on the loose, and of course Dumbledore is Harry's magical guardian and has the right to decide what he is and isn't ready for, but still... She can't say she's comfortable with it all.
Maybe it starts when Ron realises his stomach sinks whenever he gets a letter from home, before he even opens it, and that once he gets one of those letters, he'll be in a bad mood for the rest of the day. His mother seems really worried about money. He'd thought that prize draw meant they were all set for a while, but apparently the holiday to Egypt was really expensive, and then something she'd been counting on had stopped, so he wouldn't be getting any new Cannons merchandise for a while, or Martin Miggs comics, or chocolate frogs, only the usual home-made gifts, the kind that had been all anyone ever got before Dad's big promotion when Ron was nine. (Ron wasn't quite sure how that worked, since he was fairly sure he'd overheard, during one of his parents' loud quarrels over the summer, something about there were Department Heads and Department Heads, and prestige didn't make up for a pay cut, not if Dad wasn't prepared to treat this Muggle Affairs nonsense as a temporary stepping stone like they'd agreed; but the chocolate frogs were a fact.) His mum seemed kind of overwrought now, to be honest, and the way she alternated between fretting about money and insisting Ron be a good friend to Harry and look after him in her letters, it was almost as if there was some kind of connection there, even though that was clearly nonsense.
Or maybe it started before that, when Albus Dumbledore came to the Burrow in the summer of 1991, supposedly for 'tea and a catch-up' with 'old wartime allies', but managed to get Ronald Weasley alone in a completely natural-seeming fashion, and told him that Harry Potter would need a friend very badly, and that he and Ron would surely be in Gryffindor together, but perhaps Harry might find Gryffindor a little too rowdy, and would need someone to play interference, and surely Ron's experience with his twin brothers would make him the best possible person to do so.
Maybe it started a few months later, when certain people realised that the third person in the trio would be Hermione, not Neville, and Professor McGonagall's supply of patience for the boy's mishaps diminished sharply, while each of her fellow Heads of House found themselves strongly discouraged from giving out any kind of penalties for excessively lengthy essays, let alone from having little chats with her on the importance of precision; that was her Head of House's job, and her Head of House did nothing of the kind; the girl was to be fostered. Or, for that matter, when Hermione happened to mention to her parents that her summer essay for History was three times the length that was asked for, and her mother - who on this occasion was actually paying attention - expressed surprise that the school hadn't started training her out of that kind of bad habit by now, that essay-writing was an important skill, and any writer needed to develop a sense for what was important and what could safely be omitted, or only mentioned in passing. Of course she told her, with the most confident superiority she could muster, that magical schools didn't work like that, and she always got excellent marks for her long essays; it was only afterwards that she began to wonder whether this was like logic, somewhere the Wizarding World was curiously lacking...
Or maybe it starts when she starts using that logic. When she takes advantage of being surrounded by law-books, and looks up what a Magical Guardian actually is, about their duties and the limits to their authority. When she asks a Ravenclaw acquaintance whether it's normal in their house to turn in essays twice the recommended length, because she's the only one she knows who does it in Gryffindor, and the tirade she gets back tells her, firstly, that first-year Ravenclaws absolutely do do that when a subject catches their imagination, and some still haven't got out of the habit by second-year; but secondly, that it is seen as a bad, childish habit, one that they are very thoroughly trained out of using a mixture of steep grading penalties imposed by their teachers and helpful editing seminars run by older students. By third year, yes, some of them are still writing first drafts that are twice as long as they need to be, in their favourite subjects at least, but none of them would dream of handing such a thing in. When Hermione realises that the treatment she received was blatant favouritism, yes, but also that it was not helping her in the long run. And then she starts to wonder why.
Maybe it starts when she gives back the Time-Turner.
Maybe it starts when Ron writes to Bill and asks some pointed questions about their family finances.
Maybe it starts when they both apologise to Harry.
Or maybe that's where it ends.