I have an excel file were I rank my ideas based on potentiality (how much original, how much fun, how much appealing to me, etc.) and work needed (how much graphics, how much design, how much AI if any, etc.). This helps me in defining the "best ideas" (good potential and not too much work).
I write down the details of my ideas using https://app.simplenote.com.
I plan to do a little bit more planning using www.trello.com
Currently I am just learning Construct3 though, so I make some mockups of my basic idea for practice and for understanding how to use the tool.
I make (or try to make) the basic game loop and then I plan to build up from there.