I'm working on two new WIPs, both of which have large, diverse casts.
To make sure I'm not writing or reinforcing stereotypes, I've researched and read a ton about creating POC and LGBTQ characters. I'm basing most of my characters (loosely) on real-life people I've known and worked with through the years, and they occupy similar jobs or positions in the stories I'm writing, although with some tweaks and modifications for editorial purposes and greater emotional arcs, of course.
I'm really excited to write these characters. It'll be a while before I have anything for anyone to read, maybe December, perhaps even longer (I'm on Chapter 3, and have 25 planned), but it feels good to bring these folks to life on the page.
This link is recommended many times in FB writing groups, all over the Internet, and I've even referenced it with other authors while beta-reading their stories, mostly to show why we don't use foods to describe POC. It's a mistake I've made in my own writing, btw. Know it's OK to make these writing mistakes, just get educated and edit them out, before submitting to agents or self-publishing.
In my own writing, I find I tend not to describe characters' physical features overly much. While some authors (mostly romance, actually) go on about lips and eyes and etc., I tend to use social and cultural cues to show race or ethnicity and prefer instead for readers to form a picture of the character in their own minds not necessarily based on my physical descriptions. This approach comes from my 16-year-old daughter, who staunchly refuses to watch the Harry Potter movies, any of them, because "the actors don't match the images I have in my head of the characters, and the movies would ruin them." She's 16, and still holding onto her images. I honor that.
These Tumblr tags have a wealth of first-hand information / experiences to draw upon when preparing to write POC, but I get the feeling it isn't updated often and some info may be out of date.
The story I'm writing isn't a YA or MG piece, it's adult, but all of these lessons still stand.
I rarely recommend Twit for anything, but this is an exception. I found this while looking for a way to incorporate POC into my completed medieval fantasy MS in a way that was genuine. I found this resource -- the visual representations in particular -- to be priceless. Yes, there was diversity in the middle ages. Even in Europe. Especially in Europe (?). Those characters deserve places in your (medieval fantasy) stories, too.
I particularly like the advice of asking if the story is ours to tell. It's OK to include diverse characters in your story, after holding a sort of "open casting call," but it takes a lot of research and sensitivity to tell a story that isn't inherently ours. It's not that it can't be done; essentially, that's what all fiction writing is, telling a story that isn't the author's, because if authors wrote only what we know, we'd all be writing biographies. But if you choose to do this, you've got to blade the road with A LOT of research and reader feedback. The blog post also has great suggestions for getting (and taking / digesting) reader feedback on POC in your story.