Personally, I found this part of the MicroPAT test the hardest. I found the best way to approach it after a few different attempts was as follows:
Take the instruction given and convert it into the opposite direction. So if it says 10 south, convert that to 10 up. 20 west would be 20 right. and 5 east would be 5 left etc.
So as an example where 4 instuctions are given:
1 - 10 south - this becomes Up 10
2 - 20 east - this is Left 20 and combined with the last instruction the current location of the airfield is Up 10, Left 20
3 - 30 north - this is Down 30, and now makes the airfield at Down 20, Left 20
4 - 10 east - this is Left 10, and the airfield is now Down 20, Left 30.
Keep repeating the sub-total in your head after each instuction so you don't forget it.
Of course it gets harder when they start putting in diagonal instructions (se, ne, sw, nw). Remember 20 south west isn't the same as 20 south and 20 west - it's about 17 south and 17 west.
At the end of the day, this test is just a small fraction of the entire assessment.