So I have been interested in metal casting for a while and have made some blobs and large ingots, but I wanted some real results. I made some casting sand using 1 part ground cat litter (try to find one that only has Bentonite clay in it) to 9 parts regular play sand. I ground the cat litter using my mom's kitchen blender; if you do so, wash it out well or she will murder you with a butcher knife. I probably should have used fine silica sand, but play sand was what I had on hand from building my large metal-melting furnace. Regular play sand will give you coarse results. I added enough water so that it stuck together but was not sopping. I then basically followed this, but I did not add any vents/air holes for the casting. My flasks were made by cutting a frozen juice can in half; you can definitely improve this if you do this experiment. Having made my ring mold (using my dad's wedding ring), I used a soup can as a crucible and a propane torch to melt some zinc pennies (date after 1982) and skimmed off the copper coating. With a smooth action, I poured the metal into the mold. I am exceedingly happy with the results.
The image at the top of the post is the newly removed casting. I cleaned it up with a Dremel tool and a hand file and then sanded it smooth. As you can see from the two other pictures, the ring actually resembles a real ring and looks fairly nice for a first casting. It even fits on a finger and does not irritate the wearer!