I'll say pen and paper for sketching too, for it is the fastest tool to use, no need to chase the object you'll draw, or search for where to modify the font. You think and you draw, the quickest possible. But then I'd move it to a digital platform because there it's easier to manipulate a schema that you already have. Though up until this day I haven't done that. However for emacs users there is the builtin artist mode and for others who don't want to use emacs there's a program that is capable of sketches and flow charts via a script input, but I don't recall the name, sorry.

In general I usually jot down whatever comes to my mind and try to structure it all later, or on the way witg intervals. It's hard to think in a schematic way. So I'll have my notebook and pen, and write whatever comes to my mind asynchronously, that is I won't wait for the next thought. If I'll present my idea to someone else, I ll try to have at hand a schema and some notes both as brief and as clear as possible and I'll skip details for not making premature commitments.