Find a fairly level spot with no more of a drop than 2" or 3". Get some sand and level out, using two 2x4's fastened together with a bubble level to approximately level. Then drive in a 3/8" bar in the center and put a 3/8" hole in the middle of the 2x4. The 2x4 then spins about the center. You pull the 2x4 toward you, skimming sand. After you do all this, wet the sand down and compact. Repeat this wetting and compacting twice. There are lots of details on this procedure at http://www.levelgroundpool.com
Just make sure to compact well and also compact about 2 to 3 feet outside the perimeter of the pool.
Probably the best way to prepare is to hire out a bobcat and have them scrape and level your pool spot - it might cost two or three hundred dollars. You probably would want to add a few inches of sand over this to make it smooth as well. For us, we did not have access for any heavy equipment without tearing up our property - so we leveled with sand.