The easiest way to prepare a level base on soil for an above-ground pool is to use sand. Soil may need to be excavated from the site in order to ensure that the base is stable.
Get a 2x4 that is 1 to 2 feet longer than the diameter of the pool. For large pools this may need to be constructed with two or more pieces of wood using other pieces to brace each joint in place. When constructing this 2x4, make sure that it stays perfectly straight.
Place the two-by-four on the ground and use a level to make it flat. Measure the drop from the 2x4 to the ground. If the difference in each end of the pool is more than 2 inches, excavate some soil from the higher end before adding sand. Add the sand and level it out loosely.
Drill a 3/8 of an inch hole in the center of the 2x4 and drive a 3/8 of an inch rod into the center of where the pool will be situated. The 2x4 will now spin on this rod. With a level on top of the 2x4, start turning the wood and skim the sand. If the 2x4 passes across sand and there is a large area without sand, shovel more into the void and go back over the area. If there is too much excess, it can be raked off to the side.
The sand must now get soaked, either by waiting for rain or using a hose. Use a bought or homemade tamper to pack down the sand as much as possible and re-use the two-by-four to relevel it. Repeat the wetting, tampering and relevelling. It needs to be compacted enough so that when walking on it, footprints are barely left on it.