As Has been mentioned before there are only a few questions in the motiviation paper that involve mental maths and it is really just speed distance time stuff.
e.g
A B738 at 36000ft descending at 1500 ft per min has a speed of 420knts and has to divert, which of the following airports could it divert to?
Then it will give you 4 different options all different distances away and you have to choose the one it could divert to given all the information.
I reckon i failed the spatial reasoning even although I was fine with the practice questions, I think you either have that or you don't.
Best advice for that and the diagramming stuff is just think through it logically and don't panic if you can't get it just move onto the next.