Keep an eye on rhythm and phrasing–yes, rhythm isn't just for poetry–and try to make it so it could be interesting even if someone weren't reading it for the fetish content. Avoid onomatopoeia and excessive descriptions not related to the erotic or plot focus–aka don't spend more than a couple sentences describing someone's clothing unless it's integral to the fetish content or important to the plot. If someone's getting strangled with a belt or using it to escape from a peril, it's okay to describe the belt. Otherwise, something like "she was wearing a leather belt" is enough. It's infuriating to readers when they click on a story and it looks several paragraphs long, only to find that the first 4-5 paragraphs are describing perfectly ordinary clothing and only the last 2-3 have anything to do with the fetish. (Google "asstr bad curry" if you want an example of this kind of mistake.) Onomatopoeia is bad because the readers have to read each character instead of just recognizing most of the words, and because readers don't have any emotional associations with random strings of vowels. It's almost always much better to describe the sound than to repeat it. Say something like "Jen screamed shrilly," not "Jen screamed 'AAAAAAAH'." If you have to use it, keep it short: "Sally let out a soft 'uhhh' and collapsed to the floor.'" Long onomatopoeia can also break text width on some sites, or on phones. In general, it's just rude. You can do better.

Make sure to use enough punctuation. If in doubt, try reading it out loud, making sure to only pronounce the punctuation that is actually there. A comma is a connecting pause, em-dashes are a longer connecting pause, and parentheses are kind of a combination of the two (usually used when an inserted phrase would be distracting otherwise). These things are all for separating phrases, as well as clauses if a conjunction such as "and" is used. Commas are also for separating different items in a list. A period ends a sentence. It is a pause, but also means the pitch of the last syllable before it is usually lower, indicating resolution. A semicolon connects two things that could be independent sentence, but which have ideas that are very closely related; many people make the mistake of tying "independent clauses" together with a comma, but unless you use a conjunction like "and" or "but," you need to use a semicolon instead. Colons are for lists or summaries. Quotation marks are for dialog, quoting, or definitions or examples. If you are ending dialog but adding something like "he said" after it, the last period gets replaced by a comma, but other punctuation isn't affected.

There are also lots of style guides available on the internet. Look up "style guide" on google and follow the recommendations until you know what you're doing.

And yes, read stuff. Don't look for traditional books. Don't look at /lit/ either, because honestly most of it is something to be improved upon, not emulated. Archiveofourown.org would be where I'd point to, because it's got really good content sorting and also will let you sort all works tagged with "gore" by the number of "kudos" or likes they have. Though popular approval isn't completely foolproof, if you filter for single-chapter gore or guro fics in English that are most liked (yes, you can do that, AO3 is pretty much the gold standard for archive design), you'll get a good sampling of stories that are written well and can be emulated.