I doubt that tilling the ground is very common as pool prep; it would be a pretty bad idea.

The reason is, it's hard to puncture a liner that resting on a firm surface, but rather easy to do so if the liner is on a soft or spongy surface. For that reason, I'm not sure that putting the insulation panels underneath is a good idea. It might work out better if you simply raked and leveled the ground carefully, and then put a heavy duty tarp, trimmed to fit, under the pool.

One caution: leveling the ground is important with all soft-side pools, but it's exceedingly important on the larger ones. Use a water level to make SURE you are level side-to-side, end-to-end, and corner-to-corner.