In addition to the precise math that you may learn at school, and which require pen, paper, calculator and whatnot to perform properly, I find that you also need to be pretty strong in guestimating. Without any tools, but possibly using some rules of thumb.
Here's a quick example. You fly due north for a minute at 100 knots. You then fly east for a minute at 100 knots. Wind is from the NW at 10 knots. Where approximately are you now (both distance and bearing) from the starting point?