string line and level, you could start out with setting stakes around the perimeter of the area where the pool will go. tie a string line from one stake to another. they make a level that can slide along the string. once you have a level string line, then all you do is start digging and make sure you dig down the same distance along your line. then once you have desired depth around the perimeter then you work out the middle, by setting more lines or use of a larger level

Or Alternatively, you can simply spread sawdust (available for free at lumberyards) up to the level line.