One of my favorite ways to flesh out a new idea is a freewrite. Essentially, a freewrite is a block of time where you just write without any structure. For new ideas or flash pieces, these are perfect ways to stretch the writing muscle and see what you can get out of it.
Another benefit of the freewrite is that it can help you develop characters, either by splashing down a few hundred words on a slice-of-life moment for them, or by helping you see how they’d react in a given situation.
As an exercise set a timer for fifteen to twenty minutes. During this time, write down anything you want; use those minutes to expand a scene or write down a snippet of a new idea. You’re not strictly bound to the timer, but this is usually enough time to find the rhythm of a piece.
If you’re not sure what to write about, try using a prompt to help kickstart some ideas. For an extra challenge, try writing something that’s outside your usual genre or topic–mix it up with a fiction or non-fiction piece. This is also a good place to work on scenes you have trouble with, like fight scenes or emotionally-charged moments.