Water naturally creates its own level, so it is very important that a natural level is created when you make a scale model water feature. If you are making moving water, rivers or creeks, you need to tilt the surface of your sheet slightly to give the water surface the effect of running down hill. As I am creating a miniature pond, I need my water surface to appear flat and level.
Use your trimmed sheet of clear plastic to mark a line around the edges of your pool where the natural level occurs. If your foam pool is carved with a flat top, you may have a level water feature when you lay the clear sheet over the top of your pool. If your water feature is not level, use the edge of your pool surface sheet to mark the level on the sides of your pool and cut a slit into the edge of the pool along the level line. You will insert your pool surface into this slit and landscape over it to create the effect of a level water feature.