I'm constantly tinkering with various game ideas in my head throughout the day. When an idea starts to really come together in a satisfying and interesting way I start writing it down and then seeing if I can fill in the left over details that I realize I hadn't considered. Usually I can't or nothing that really feels good comes to mind so I set it aside and if I think of those things some other time I'll come back to it and finish the write up.
Eventually I have a few fleshed out designs I really like and when I have the tiime and energy I'll pick one of them up and start trying to implement them.
At some point one of the projects will generally pick up more momentum than the others and will become my 'main project' for awhile, but usually it will run out of steam just after the point where it's a playable prototype.
After having a few playable prototypes, sometimes I'll get excited again to pick a project up and push it farther.
Super rarely a project will actually go far enough that it's functionally done and just ready for polish and testing. At this point I have to stop being so whimsical and focus down on getting the 'boring' bits done since There's never going to be a time where I just feel like doing those things so putting it aside would mean its just going to die.
Later on I publish the game when I'm sick of working on it as long as it's in a presentable state.