Create the original design for your medallion by sculpting it in clay using any sculpting tools you have available. Use toothpicks and small knives if you don't have access to proper tools. Keep the design simple and on one side of the medallion. Make sure there are no undercuts, meaning that any raised portions of the design should not have indentations behind them, but rather raise from the background at a 90-degree angle, or more. Allow it to dry completely.
Place the hardened medallion, sculpted side up, in a cardboard or plastic container that allows 1 1/2 inches of space around the medallion's edges. For example, if you have a round medallion that is 3 inches across, use a container that is at least 6 inches across.
Coat the bottom and sides of the container, as well as the medallion, with petroleum jelly. Keep the layer of jelly thin on the medallion. This keeps it from sticking to the mold; a thick coat will obscure the detail.
Put on latex gloves and measure out enough plaster powder and water to fill the container to a depth of 1 1/2 inches beyond the highest point of your mold. The plaster packaging will tell you the proportions of powder and water necessary.
Pour the water into a mixing bowl, then use a flour sifter to quickly sift the powder into the water. Stir it by hand, using your gloved fingers to gently break up any lumps. Try not to agitate the mixture too much or air bubbles will form, but work quickly so you finish before the plaster begins to harden. Do not use the flour sifter for food after this.
Flick the plaster mixture onto your clay medallion a little bit at a time---the velocity of flicking it on reduces air bubbles. As soon as the face of the medallion is coated, pour the rest of the plaster mixture into the mold before the mixture hardens. Allow the plaster to dry overnight.
Pry your clay coin out of the mold gently and wipe away all traces of the petroleum jelly from your plaster mold.
Place your pewter ingots (bars of solid metal) in a fireproof container, known as a crucible. Steel or cast-iron pots work fine for pewter-casting crucibles, but do not use nonstick-coated cookware.
Put on heatproof gloves and safety goggles and turn on your butane torch. Circle the flame constantly over the metal, keeping the heat even and constant until the pewter melts. Impurities (called slag) will float to the top of the melted pewter. Skim these off with a steel spoon.
Shut off the torch and lift the crucible with fireplace tongs. Slowly pour the melted pewter into your mold until the metal is even with the surface of the mold.
Pull the cooled medallion out of the mold. Pewter shrinks as it cools, so removing it from the mold should be simple after an hour or two of cooling time.