Casting makes up only one part of a process known as moulding and casting, in which you mould an object before you can reproduce it. Using the correct mould plays a vital role in producing good casts, so when casting metal in rubber, use only high-temperature moulds, as well as metals formulated to allow melting and pouring at lower temperatures---such as pewter.
Things you need
Use a number of specialised tools for casting metal in rubber moulds. You probably already have some on hand, such as mixing bowls and utensils, but may only find others in speciality stores or online---particularly metal working tools.
Use a high-temperature RTV rubber (RTV stands for room temperature vulcanisation) to make your mould. RTV rubber becomes solid at room temperature and does not require the prolonged heating that used to play a part in the mould making process. Purchase RTV rubbers in convenient kit form, or in quantity as two-part urethanes or silicones. Formulations vary---read the manufacturer's specifications before purchasing.
Melting metal requires both a crucible to hold it and a furnace to melt it, as well as steel spoons for stirring. To a certain extent, you can reduce these tools to simpler forms for melting low-temperature metals---use a heavy stainless-steel or cast-iron pot for a crucible, and a hotplate or stove capable of reaching temperatures above 280 degrees Celsius (550 degrees Fahrenheit). Melt the metal---purchased in small ingots---in a pot on your stove, saving the cost of expensive equipment. Use only leather or well-padded heat-resistant oven gloves to grip the handle of the pot.
Use metals such as pewter and other "white metal" ingredients that make up many of the low-temperature or fusible alloys (bismuth, antimony, zinc, copper, etc.) for this method.
Prepare moulds by washing and drying thoroughly. Dust moulds with talc to help metal flow more easily and prevent sticking when the time comes to release the cooled casting.
Ensure that you have everything you will need from starting the melt through pouring the mould on hand and ready before beginning to melt the metal. Never leave a pot of melting metal unattended.
Place talc-dusted moulds on a sturdy, safe surface---preferably ceramic, metal or wood (definitely not plastic) within reach of the metal melting area to avoid carrying the very hot molten metal any distance. Metal cools rapidly as you remove it from the heat source; pour it as quickly as possible. Pour your metal on a table at knee-level, or lower, to facilitate pouring and lessen the chance of injury from back splashing or spills.
Warning: Molten metal has some inherent extreme dangers; do not underestimate the potential for injury. Always wear a fire-resistant apron and gloves, goggles or foundry mask, and heavy leather work boots. Keep children and pets well away from the casting area.
Put ingots into the pot, and place the pot directly on the hotplate or stove until the ingots melt. Move immediately to moulds, pouring slowly from one side to reduce air bubbles in moulds. Allow metal to cool completely before handling. (Even after the mould skins over and appears solid, the core will remain molten for some time. Be patient.)
De-mold and examine the casting for flaws. Fill large bubbles with small rods of the same material---insert and cut flush with a hacksaw or wire cutters, then pound into carefully with a ball-peen hammer to lock it into place; file smooth. File rough edges with small files of assorted shapes. Polish, paint or patina as desired.
Use metal-coloured epoxy (such as plumber's epoxy---made to resemble white or copper alloys) to fill minor holes and other flaws, only if you will not subject the metal to heat.