Given how few words you have, when you are writing flash, you tend to go with the familiar. Whether that is a contemporary setting, or a science fiction or fantasy world so full of tropes the reader is clear on where they are as soon as the story opens.
If you are writing fantasy there will be some swords, possibly dragons and magic in the first couple of paragraphs. Equally, science fiction flash will refer to the space ship, planet or alternate reality upon which your characters find themselves; ‘Gee it’s cold here on planet X’ sort of thing.
The other thing you might do is use familiar tags on your characters; the geek has glasses, the action heroine looks like Lara Croft, the bad guy has dark hair and a patch on his eye. I don’t advocate that sort of thing, but it can save you a lot of words when you are trying to build your story but keep it under 1,000 words.
Flash also lets you use big aspects of the story scene, such as the speculative element, to actually be the punch-line. Because it is so short the reader can hold all of the story in their short term memory, so you could do your big reveal at the end and have the reader reflect back on the story with an a-ha, recognising all your carefully hidden foreshadowing when you finally reveal they are in space/ underwater/ on an island made out of marshmallows/ the story is being told from the perspective of a dog.
I don’t tend to do that kind of reveal in my stories, but my story Random Impulses is probably the closest that I do get to doing this. This was actually my first ever flash fiction story, and it was my first published-by-someone-I-don’t-know story back in 2001.