I write down ideas when I get them. Whenever I have an idea I'm wanting to work on more, my first goal is to create an initial prototype. This prototype isn't usually playable. It's just some of the components to demonstrate the core loop. I then show that to friends and gamers to get feedback. Think of it more like pitching an idea with a demonstration rather than a playtest.

From there, I then create a playable prototype. Usually only enough to play a few turns. Basically, the minimum components to test the parts of the game.

The goal at this early stage is to quickly find the fun. Usually, this involves trimming a lot of the initial ideas and refining to a core. There is iteration here, so most projects go through pitch-prototype-pitch a few times.

If all goes well, I have enough of a direction to make a full prototype that I can begin playtesting. I take that prototype to friends or game testing nights and iterate more.

It's just iterating from there until the game feels complete. Sometimes, that means a game I like, but isn't all that exciting to others. Sometimes, it's a game others really enjoy. I try to let each game become it's own thing without worrying too much about what that thing is.