Dan,
No different to learning to fly .... mainly a matter of confidence and practice ...
(a) demonstration .. the teacher (instructor) does the sum on the board (demonstrates a landing)
(b) practice .... you try the same problem and then with different numbers under the supervision of the teacher (instructor might follow through on the controls and then you do it yourself with the instructor monitoring)
(c) independent practice .... you do some sums on your own (solo circuit practice)
(d) competence ... you can do any similar problem with confidence (you get your PPL)
Stick with it, buddy .. just a matter of thinking about it, practising it, gaining confidence and competence in doing it.