I would suggest one of two methods - 1. Rent or buy a Roto-Tiller, which will church up the lawn, which you can easily level out with a shovel after. If you leave the bits of grass in it, you should have full grass there again in a few months, especially with water being splashed all over.
Method 2. would might help better if you have a lawn that is flat on one portion, then slanted on another portion but not at a steep incline, and what you can do is to find some free "fill dirt" or buy some soft soil and use it to build up the flat portion until you have a large enough surface area to hold the pool.
In my experience, the tiller is the better method, which costs much less in money and time. Though the second method can be made to look much better, and if you even out the whole lawn by filling it in, you'll add value to your home. So there's always that silver lining, lol =)