So, you've had a brilliant idea that you can't wait to develop!
You've setup your code repos, development environments and all the infrastructure you need to write your software.
What's the very first thing you tackle when you start to write code? User Stories? Use Cases? Database Design? API?
I am beginning to write a new web application that am extremely excited about and was curious to know the thought process people use to approach an idea.
For my solo projects I just hack. I would recommend the following highlights (for me personally). On your solo project you can now DO all TODOs. Litter the code base with those little buggers. TODOs can make a very effective replacement (or supplement) to bug/task tracking systems.
Make sure you have a fast build. I will make the assumption that you are writing unit tests and say, optimise those so they whizz by after new feature is coded up. When they get slow, refactor them like a monster until they are fast again.
All in all (and the list above is really specific to me) you can now optimize for your development pleasure. Do this! That is the best methodology I know.