It's easy if you keep in mind there are only 180 degrees in a triangle, so if you know the value of two angles you can easily find the third one. And it's even easier if you consider that you count angles always in a clockwise direction relative to the North being considered—be it Grid North, True North, Compass North or Magnetic North.
Simple, not precise sketches will do it for you, keep it simple. And on some occasions if you draw the sketch accurately with a divider and protractor you'll get to the solution without doing any fancy calculation on your calculator. While I was quite worried about Gen Nav at the beginning of the syllabus, it turned out to be one of my favourite subjects.