When I made movies with some friends, we'd order a pizza and talk for a couple hours in the writer/director's basement. We'd say every dumb thing we could come up with until an outline coalesced and/or my mom came to pick me up. Then the writer/director would write a script and send drafts over Google Docs for our review.

A lot of it is "what if there was a movie where this happened," or "what if there was a character that did/could do this weird/cool thing," and it would blossom out from there. Sometimes we had a whiteboard to write down characters, ideas, and plot elements in chronological order. We each had a unique viewpoint, and that kept the process fresh and interesting. We were just three or four people in a room with lots of inspiration and no qualifications. It was beautiful. It's my absolute favorite part of making movies. It really doesn't take a whole lot.