And dont dig below the neighboring area too deep or water run off will collect around the base of the pool; possibly wash unwanted debris under it, like pebbles or twigs.
Mine, I used a 10' pressure treated 2x4 (they stay straight) and a laser level. Oh, and a shovel :00000060:
I put a stake in the center and ran the board around in a sweep with the end of the board on the ground, at the stake (like the second hand on a watch). And just worked the high spots to the low spots with the laser level. And I have two layers of tarps under it; mainly because the shells fragments in the sandy soil here feel like they are going to poke a hole in the bottom when they work their way up under it. When I got the pool up, the water line is even all around the top.
If you havent invested much into it and have some $ to spare, check out a saltwater chlorine generator system. Its unreal. How many gallons (or dia and height)?