Start by writing ADRs, to describe your thought process, you can do so in google drive sharable .word files.
Here's a nice vid on database tables:
Here's a great guide I found on system design
This is a very good video I like on system design in general
And a more beginer friendly free course by freeCodeCamp.org
Use excalidraw when you draw your system design
Here's a guideline i wrote on how to do system design
And some design document examples:
Some additional tips: