FIRST, consider your character's sexuality in context.
You’re writing a LGBTQ person, but there’s a huge variation of what exactly that looks like. Did your protagonist grow up in a mostly-accepting or mostly-unaccepting area? Are they out to everyone, to just friends, to close family, to no one at all? Do they have a connection to other LGBTQ people or no?
Most of these things won’t explicitly come up in the story--many won’t come up at all--but regardless they’re a good idea for you as the author to understand and have thought through, because a closeted gay man from a strict and homophobic family who’s dated multiple women in the past but never so much as held hands with a man is going to have a very different way of approaching romance than a happily-out man living in NYC who’s had many relationships before and marches at Pride every year.
(This is where I feel “just write someone who happens to be LGBTQ” advice often falls short, BTW, because while well-intentioned it often ends up with characters who feel more like a collection of randomly picked traits than a real person whose life events and experiences shaped who they are. If you're writing a book all about a doctor's experiences in a hospital, you wouldn't just write someone who happened to become a doctor - you'd consider their motivations towards pursuing medicine and feelings about being a doctor as well as the non-doctor parts of their personality.)
SECOND, be aware of stereotypes but not terrified of them.
However much you try, even if you’re very pro-LGBTQ it’s really difficult to exist in our modern landscape and not subconsciously pick up some stereotypes. (Not just for straight people either, LGBTQ people do this as well.)
When you read your finished story through for the first time, keep one eye out on how you’re treating your LGTBQ characters as you're editing. Look for patterns in how they look/behave/feel, and make sure you're okay with all of those and what they say about how you as the author feel about those characters. Is every gay man promiscuous/totally sexless/feminine/ultra-masculine, etc. etc.? Do your lesbians all hate men? Are your bisexuals all promiscuous?
On the other hand, I’ve seen it sometimes that writers with the best of intentions realize stereotypes exist and immediately try to get rid of them to the point of wrecking their own work: can’t have a villainous gay person, that’s a stereotype! Can’t have a gay man wear make-up, that’s a stereotype! Can’t have a lesbian be masculine, that’s a stereotype! And so on and so on, completely ignoring that many people actually have those traits in real life and trying to flatten the broad spectrum of how real people are into a narrow slice of what’s acceptable to write is bad writing.
In reality, LGBTQ stereotypes are so weird and widespread and often contradictory that the only way to never include them would be to write a bland useless nothing of a character with no personality traits and no interesting features. Rather than focus on eliminating stereotypes from your writing entirely, focus on making your characters interesting and human and varied enough that their traits seem natural and obviously a part of their larger personality to the reader.
THIRD, do some research, the same way you'd research anything else you're interested in writing about.
The old adage “write what you know” doesn’t mean “never step outside your comfort zone”, it means “get to know what you write about”. In the same way you’d probably do some research on how a courtroom works before writing a legal drama, you want to do some research on LGBTQ perspectives when writing an LGBTQ romance.
Note that this does not mean “read a hundred memoirs cover-to-cover before you even DARE pick up a pencil, you ignorant fool” - this can take the form of asking your LGBTQ friends for feedback on your story, checking out LGBTQ artists/comedians/authors/poets/cartoonists, researching where and when your characters would have been born and what attitudes about LGBTQ were often like in that time and place, etc. etc. Make it a casual thing, don’t worry about doing Important Learning so much as just broadening your horizons a bit and getting a feel for how different people view the world.
(And, as a bonus, these are all things that will improve your writing even if you never write another LGBTQ romance story again in your life. Getting feedback on your work is one of the best ways to improve, getting inspiration from lots of different artists and creators will help you develop your own style, and research is always the sort of thing that comes in handy when you least expect it.)
Good luck on your story!
