People have been wearing cast jewelry for over a thousand years. One of the most traditional methods of creating a mold to cast gold rings out of is called the Lost Wax method, where the ring is sculpted in wax, molded then heated to remove the wax, leaving an empty mold behind. This technique is still used today to create jewelry of all kinds.
Sculpt the ring out of jeweler's wax. You can purchase wax that is already in tubes of various sizes, which only require you to cut off a slice, smooth it and carve in any details.
Add three 1/2-inch sprues (small wax cylinders or sticks) to the outside of the ring with a bit of melted wax. They should be positioned to create a Y shape with the ring in the center. The bottom sprue will attach to the base, while the top ones are for pouring in the melted gold and allowing the air to escape.
Attach the ring form to the metal base by the bottom sprue. Use melted wax as glue.
Attach a plastic container to the metal base with hot glue. It should be only slightly larger than the ring form, with no more than 1/2-inch clearance on the sides. Cut the top off to form an open cylinder if needed.
Mix a batch of plaster and water according to the directions on the plaster's packaging.
Pour the plaster into the container, completely covering the ring form. Stop pouring when the top sprues are almost, but not completely, covered.
Tap the sides of the container to force any trapped air bubbles to the top.
Allow the plaster mold to dry and remove it from the container.
Put the mold into a microwave and melt out the wax, leaving an empty mold. The time required will depend on the density of the wax, the thickness of the mold and the power of your microwave.