Casting metal sometimes seems like a lost art, but in reality it is a fairly simple process but you will need to know more about it before you can do it competently. Understanding how to cast and the techniques used are important steps in becoming a backyard founder.
Make your sand. Use only clean silica sand for this step. Some so-called "sand" is merely finely-ground byproducts that will be unsuitable for casting (for instance, sandblasting sand is typically made from slag, which is iron foundry waste and is made of carbon and iron as well as sand). You will want the finest grain to get the smoothest cast possible, so you may need to filter the sand particles using a wire mesh and a bucket. Mix the sand with a casting clay mixture and some water (if necessary) to that the casting sand is damp and clumps well when squeezed tightly.
Press the shape. Place your shape on a wooden board next to a molding tube (a long, circular metal tube or cylinder that varies in size depending upon the size of the molded product). Place your casting frame around the shape(s) and tube on the board. Fill it with sand, packing tightly. Remove the tube (or "sprue"), flip over and remove the molds. Pack the other half of the casting frame with sand. Place over the first half and secure. Turn over so your sprue (casting tube hole) is on top; this is where you will pour your metal. The part with the mold shape in it is your "drag" and the other with just sand is your "cope." Be sure that they are flush against each other and weighted down to prevent spilling the molten metal.
Melt the metal. Placing your metal in the crucible, place the crucible in the furnace. Each metal or metal alloy has different melting temperatures, so be sure to heat your metal to the appropriate temperature. For example, the melting point of copper is 1083 degrees Celsius while the melting point of lead is 325.7 degrees Celsius; some common alloys such as bronze vary depending upon the tin-to-copper ratio, but are typically between 850 and 1000 degrees Celsius. Ensure that your metal becomes red hot before removing it from the furnace since heating, cooling, and reheating the metal will allow gasses to infuse the metal and may result in a "caved in" casting.
Pour the metal. The molten metal should be red hot (literally) or it may cool too early and make a poor cast. Be sure to use the proper safety equipment to protect yourself and do not cast near flammable material. Once the metal is poured, wait an hour for it to cool. Break apart the casting frames, remove sand. You will have a rough version of the shape you are casting; you will need to further refine it (through polishing and sanding) to prepare a final product.
Casting any metal is extremely dangerous and can cause a fire almost anywhere. Caution is advised.