It's a fairly long process for me:
1. I get a game idea. I usually work out some of the main details in my head, then wait a few days before revisiting the idea. If I like it, great! Onto Step 2. Otherwise, back to the drawing board.
2. I make a document listing out the details of the game (for example, my Smash Unbounded document has the characters, items, stages, etc. I planned the game to have). I don't usually have a specific order to work on individual pieces, but once I start working on more story-heavy games, that'll usually be the first thing I work on. During this phase, I'll also sometimes get some feedback on the Fantendo Discord or from some of my other friends.
3. I start writing out what I actually want to say on the page itself.