The way I do it is that I whenever I start working on some functionality and it’s not immediately obvious how to implement it, I open a text file and write down my thoughts as something between stream of consciousness and design document, usually formatted as a multi-level bullet-point list.

I start with what I am trying to achieve and list the different design approaches I can think of, adding advantages and disadvantages of each one as they come to mind. By the time I’ve written down all my thoughts on a design decision, it is often clear to me which approach I favor.

This can be repeated for more and more detailed aspects of the implementation (e.g. “which function should this be added to” or “what to name this function/struct/variable”) until I feel like I can come up with the remaining details as I’m writing the code. If I get stuck somewhere later on, I can always go back and add more details in the text document.

For larger or more important features, this list can be cleaned up and become documentation or perhaps a comment somewhere, but I often find that the writing is a useful tool to get unstuck and to clarify my thoughts even if I end up never reading it again.