Green sand that actually holds a shape: the ratio and the squeeze test

Play sand is too coarse and too round. You want fine silica (100 to 120 mesh) or, easier, buy a bag of petrobond (oil-bonded, no water, great detail, lasts forever) for about $60.

For homemade green sand: 10 parts fine sand, 1 part bentonite clay (sold as clumping cat litter, grind it to powder), water a little at a time while mixing. The test: squeeze a handful. It should hold the shape of your fingers and break cleanly in two when you snap it, not crumble and not smear. Too wet makes steam and blows the casting full of holes.

Ram it firm around the pattern, vent with a wire, dust the parting line with talc or parting powder. Sprue, riser, gate. Then the fun part.