“Write what you know” is one of those pieces of writing advice that everyone’s heard of. People either love it, hate it, or completely misunderstand it – sometimes wilfully. You get those who point out that JK Rowling didn’t really go to wizard school before writing the Harry Potter stories – or (depending on the literary taste of the person putting this idea forward) that Chuck Tingle hasn’t really had sex with a sentient gay ironing board/a space dinosaur/handsome sentient food. Someone will usually suggest, when this topic comes up, that if people only “write what they know” then the world will be full of novels where someone goes to the shop, has their tea and watches the telly and… er… that’s it.
Oh, and that favourite standby of people who want to annoy erotica writers – “So, have you done all those things you write about?” Can we settle, once and for all, that there is a difference between knowing about something and having directly experienced it yourself? Thank you.
Much fiction is fantasy-based in that it features things that no one has directly experienced – space travel, time travel, vampires, superpowers. At the moment, though, some people are getting rather more hung up on fictional people than 
fictional experiences.As in: can you write about a character who is massively not like you in terms of gender/ethnicity/abilities etc? and if you don’t have any characters who are Not Like You, is that also wrong? Will you get battered to bits on social media one way or the other?
It isn’t actually as difficult or frightening as it might sound. It’s predominantly a matter of not being lazy. If you are building a character with a very different set of physical/social attributes to you, you need to do your research. You’ll have a bit of a head start if you know people in the relevant demographic already, particularly if you are the sort of author who is hugely nosy – I mean, observant. Though be careful about assuming that your friend or family member’s experience of class discrimination, wheelchair use or practising a minority religion is the default experience for everyone who shares their ethnicity, neurodiversity or sexuality. For flash fiction or anthology submissions with a word limit of under 10k, you won’t have a lot of room to put in an entire backstory for any character, but you need to know the backstory yourself. Do your research. Read blogs, books, Twitter threads and articles on relevant topics – this one got a bit of attention recently.
“Write what you know” isn’t your only option. Skipping your research to the extent that you rely on tired tropes, dumbfuck stereotypes or a bit of ‘diversity’ boxticking where a range of characters are wheeled on just to represent their social, cultural or physical-ability group are the options you should avoid.