I wouldn't put a frame pool on sand; the sand will shift away from the legs.

Dig to level, measured level, a straight board with a torpedo, 2' or 4' level would work but a water level with a couple marked stakes would be even better. After the site is level, a couple layers of tarpaulin under the pool should be good.

Sand is often used inside the walls, under the liner oftraditional steel wall AG pools. The pool is built on level, undisturbed ground.

I can't tell you if 11x20 is enough; are the instructions silent on the site dimensions?