One thing I'm doing since a few months is creating a fresh directory for every side project I start. Even if it's just "I wonder if I can (easily) extract all course information from this really slow school website that only offers a search bar" (it appeared to be easy). For a bit of structure, each directory starts with YYYYMM and a dash, and a few keywords that remind me what next side project I just started.
Inside the directory, there should be at least one text file containing steps I took, ideas I had, or URLs I visited, to provide my future self a nice amount of context whenever I'm revisiting that side project. For the course information side project, simply documenting the steps I took in the browser while visiting the school's website and just pasting the 'Copy as cURL' command for a certain request inside that file is sufficient! (I'm aware there's a lot of knowledge hidden in that previous sentence; it's more an example of writing down what steps you took while tackling the problem)
The directory listing, over time, should also give a nice insight of all the things I've discovered, tried, or perhaps even finished.