Soul Bond fics aren't inherently bad, they just have common pitfalls. If you're thinking of writing a Soul Bond fic, then check out this short essay by WhosThisGeek.
“Perfecting the Independent Harry Genre” is a worthwhile read for anybody who wants to write an Indy!Harry fic. Much of it is a monologue by Taure, but he's not the only person to have usefully contributed to the thread.
Wixen acronyms typically have full stops after each letter: “O.W.L.” and “S.P.E.W.,” not “OWL” and “SPEW.” Since Pottermore, however, Rowling seems to have dropped the full stops: ICWQC, MACUSA, et cet. This might reflect a larger stylistic shift on her part: in the Foreword to Quidditch Through the Ages, Rowling (as Dumbledore) acronymizes “United Kingdom” with full stops. My preference is for full stops, but the important thing is just to be consistent about it, or at least consistent about your inconsistencies (e.g. maybe American wixes don't use full stops, just like they apparently spell “Legilimency” with an “s”).
I don't want to include a list of proper spellings for every character's name, because that would be boring and do nothing that a Google search can't accomplish, but there is one character in particular whose name is consistently misspelled, and I would like to set the record straight: the name of the attractive Slytherin boy with seven dead fathers and an invitation to the Slug Club is “Belize Zambia.”
Nobody abbreviates Defense Against the Dark Arts as “DADA,” or, well, anything else, either. I prefer “Defense” as a shortened form, but it's just as non-canonical as “DADA,” so do what thou wilt.
Fudge is the Minister for Magic, not the Minister of Magic.
Capitalization is something which many—perhaps most—writers get wrong about Harry Potter terminology, though it's also something that few, if any, readers actually care about, so long as the writer capitalizes all of the normal stuff, like character names. (“Dark Lord,” incidentally, should be capitalized)
The most consistent capitalization rule applies to spells: specific spells, like Hot-Air Charm and Killing Curse, should be capitalized, but categories like “charms” or “anti-intruder spells” should not be capitalized unless you're talking about the class called “Charms.” Incantations, and the term “House,” should be capitalized. Hyphenated spells and potions capitalize both parts of the compound.
Most objects and many beasts have capitalized names (Invisibility Cloak, Diricawl), but there are exceptions (manticore, sugar quill). The general rule appears to be that terms which Rowling didn't invent (centaur, goblin, wizard) are uncapitalized, while words of her (and your) invention should be capitalized. This isn't completely consistent, however, because Invisibility Cloaks are hardly any more original to the series than broomsticks and flying carpets, and sugar quills are at least as innovative as Cockroach Clusters.
The Harry Potter Wiki universally capitalizes the names of beasts (Manticores, Sphinxes), which is a defensible position. Their universal capitalization of Veela, when the names of other beings are left uncapitalized, is less defensible. The names of beings (giant, vampire, veela) are not capitalized in the books, and it should be noted that “veela” is both the singular and plural form of the word. The term "dementor" is not capitalized either.
The words “Half-blood,” “Muggle-born,” and “Pure-blood” should be hyphenated and capitalized as shown. “Muggle” and “Squib” should be capitalized. The words “witch” and “wizard” should not; if you use “wix,” then this probably shouldn't be capitalized either. The words “half-breed” and “part-human” are not capitalized. Professions like “Auror” and “Healer” are capitalized. "First year," "second year," and so forth are neither hyphenated nor capitalized.
Overall, the series capitalizes words more often, and more extensively, than native English speakers are used to, but it's not unheard of and there's room for more extensive capitalization if you desire. “The rise and fall of capital letters” is a short (500 words) but serviceable introduction to older styles of capitalization in the English language.
Spells should rarely, if ever, be referred to by incantation rather than by their proper names. Canon violates this rule, but there's no reason that a paranoid fuck whose paranoia has earned him a nickname would ever tolerate a bunch of schoolchildren (who might be Death Eaters using Polyjuice, for all he knows) to recite the incantation of the Goddamned Killing Curse, so I think that this is just something that Rowling didn't think about. Somebody might say “the Leviosa,” I guess, but saying “the Avada Kedavra” should be a major faux pas, on the level of pointing a gun at someone. Please, don't have your characters do that, unless they're honestly that dumb and careless.
It is also worth noting that there are no canonical spells with incantations longer than two words. I think that's an easily-breakable rule, if you want to convey that this spell is very complex or something, but be aware that you're breaking it and take at least a solid minute to think about whether there's something more interesting that you could do than just make the incantation longer.
I recommend that you do not add unnecessary apostrophes when describing a spell, and (generally) rely on the name in spell in order to figure out an appropriate construction: “Blasted” and “Cruciated,“ not “Confringo'd” and “Crucio'd,” nor “Confringoed” and “Crucioed.” I say “generally” because there are some cases where the incantation definitely seems more appealing, e.g. “Obliviated.”
The plural of “Patronus” is “Patronuses.”
Wixes certainly cast spells, but do they also…
Manu86's “Dialects and Accents in HP ” breaks down the speech patterns of Rubeus Hagrid, Stan Shunpike, Olympe Maxime, Fleur Delacour, Viktor Krum, Winky, Dobby, and Kreacher. Wayback Machine Backup here. "English to Hagrid" is sufficient for a quick and dirty "translation" job.
If you're going to write a Quidditch scene, then you should probably check out the second half of The United States of Accents (the section titled “Sports On The Sentence Level”). Even if you don't have an announcer for the match, some of the suggestions still carry over; a sports announcer's job is to make a match entertaining to hear, just as yours is to make the match entertaining to read. I also recommend that you listen to announcers for a sport that you enjoy, without (and this is crucial) watching the match.
My readers seem to like how I write duels, especially in “Out of That Story” and a particular scene from “Hermione Granger and the Silent Country.” Here are a few principles that have helped me.
When somebody says that you're following the Stations of Canon, what they mean is that you're hitting all the same major notes as the original books. This can be rather general — you've got the Philosopher's Stone in Harry's first year, the Chamber of Secrets in his second, etc. — or so specific that you're mirroring the books down to individual scenes and chapters. Generally speaking, people prefer to not read “the original books, except it's Daphne Greengrass brewing the Polyjuice this time around,” so it's worth thinking about how to avoid this.
The obvious tactic is to just…not do that shit. Let things diverge , for Merlin's sake. Really think about whether Voldemort would be gunning for the Stone in your timeline or if he might be up to another dastardly plan. Heck, he might not even be involved in the 1991 - 1992 school year. If you've got an interesting enough AU, then why not let the readers just enjoy your universe for a little bit. TheSinister_Man's Prince of Slytherin series is nearly 1.3 million words long at the time that I'm writing this — your readers are not going to complain about More Words if they like words, and you're not printing this out, so you don't have to worry about the number of trees you're killing or whether it's even physically possible to print the book that you're writing (though you should probably worry about whether you're ever going to finish the story at the rate you're going). If you think that your AU won't catch the reader's interest, then maybe you should go back to the drawing board.
Another thing that you can do is knock events askew. In “Certain Dark Things,” by rentachi, this works a couple of different ways. First of all, the public thinks that the Laddie Wha Lived is Neville Longbottom, so most of that canon shit is happening to Neville while Harriet stays out of trouble gets into a whole new brand of trouble. I'm greatly amused every time we're reminded that Neville and his pals are off doing Something Vaguely Stationary (don't tell me that's not the right word!) and it even reinforces that all these other people are the protagonists of their own lives and have their own shit to deal with. The second thing that rentachi does is make events rhyme rather than repeat: in Harriet's second year, she does still get involved in Polyjuice, but the circumstances are notably different. In another story, which (to avoid spoiling those who want to remain unspoiled) I won't name but will link here, the trio brew Polyjuice in their second year and then Tom!Ginny notices — he does frequent that bathroom, after all — and fucks it up.
Finally, if you've just got to have things stay the way they were on the go around, then just skip the bits that stay the same. Lots of fics do this from scene to scene, but the trophy goes to “The Never-ending Road,” by Laventadorn, which says that you know this shit already and just fucking skips the whole first year and fills you in on any divergences while you're skipping along through Harriet's second year. Figure out when the interesting shit is going to happen, then start from there. Another good example of this principle at work is “Witch Hazel,” by Wise_Himmel, which doesn't skip nearly so far (we begin shortly after the Welcoming Feast) but still goes further than most stories do.
(While we're on the topic, it's worth pointing out a station that most fics don't stop at: In each of the six books where Harry gets a new professor, he always meets them before he arrives at Hogwarts: Quirrell at the Leaky Cauldron, Lockhart at Flourish and Blotts, Lupin on the Hogwarts Express, etc.).