the few i have built around our house, it was more trial and error, the hardest part is getting enough dirt piled up, shape it a little bit like you think it should look, walk it in real tight (take the tractor/bobcat what ever... and just drive back and forth over and over and over, over lapping your tire tracks a little with each pass to pack it all in really hard)
once you get it shaped a little, get on your quad and give it a slow roll over, hit it a little faster, take note of how it throws you, do you want it higher? longer? faster? etc. its just trial and error till you figure the shape you like.
for some reason when i build a jump ( might be the same for some ppl) but i just knwo where to place the landing, its really weird. Like i can see a jump and invision where your gunna land and im like 99.9% right