This step applies to any type of model pool or pond, regardless of whether you pour, cast or layer it.
Take a piece of foam the size you want your base to be and carefully trace around the base of your pool to mark where you need to carve out a recess for it in the foam. Make sure your layers of foam are deep enough to carve out the base. If you have thin layers of foam, it is often easier to carve out successive layers, then glue the final shape together when you have trimmed out the centers to hold the miniature pond.
Once you are comfortable working with various materials that resemble water, you can build up your terrain and pour your water material directly onto it. As this is a practise project, using a mold and carving a space for the molded pool works better than preparing the base and attempting to pour the material into it.
E-Z Water is fairly brittle when set, so if you prefer, you can break apart your molded water to make irregular sections for smaller ponds or puddles. If it breaks when you try to fit it into your foam recess, don't worry, just take a broken piece and adapt it for a free form pool.