I use the concept of demo days - where I demo something to myself. Some people would think of it as a sprint, but a demo enforces that it is something I can act

I use the concept of demo days - where I demo something to myself. Some people would think of it as a sprint, but a demo enforces that it is something I can actually show. Like "I can launch the program and have it say print 'hello'". In my experience this works much better than stages or percentages. Lots of things just fall out. A demo that has bugs is not a demo for example. I use a stack of demos, each as a 3x5 card. Low overhead, easy and quick to modify, etc. As a solo dev it is was too easy to get lost in the process and tools, but this works well for me.