Mix up the mortar to the consistency of cookie dough, slowly adding water to the dry mix as needed. Mix the mortar thoroughly, let it sit for about 3 minutes, then remix, adding a dash more water if needed. Coat the plastic form with cooking spray. After filling the container, give it a few quick shakes to settle the mortar. Then form a mound using a spoon or small trowel so it resembles a loaf of baked banana bread.