Building your Coal Forge

You will need:

Some sort of heavy duty steel drum that will be your firepot. NOT GALVANIZED
A length of pipe to connect to the blower. NOT GALVANIZED
A adjustable blower or some bellows. If you have no adjustment for your blower, use a butterfly valve in your pipe to limit the airflow.
A fuel source, in this case, coal. (If you can find a supplier, use lump coal or charcoal, not the pressed charcoal briquettes.)

You don't want to use galvanized steel of any type, because when it gets up to forging temperatures, it emits a toxic gas that will happily remove your life.

You can use an old brake drum for your firepot, and drill a hole in it to fit the length of pipe.
Secure the drum onto a wooden frame using bolts, make sure it is very secure. If it falls apart when you arw working, 2500 degree coals all over your feet is not a good idea.
The pipe should fit snugly in the drum, and should extend about 1/2" into the firepot.
Connect the blower to the other end of the pipe in a way where you can easily adjust the amount of airflow.
Put a goodly amount of coal into your forge, enough to have a dome shape on top, and light it how you would light a fire, or however you usually do it.
when the coals are going, turn on the blower and adjust the flow rate so that the flames exiting the coal pile are about 2 inches high. After a few minutes, the forge is hot enough to use.