Clean an old decrepit cast iron pan by placing it in a self-cleaning oven on the shortest cycle. The high temperature will burn off any food bits and grease that have been caked on over the years. Expect the burning process to produce some fumes so keep the stove exhaust fan running.
Cool the pan slowly in the oven for an hour or more. Once it has cooled enough to handle, coat it with a thick layer of lard. Cover every part of the pan, including the handle, using your hand or a rag.
Bake the pan at 500 degrees for an hour to allow the grease to carbonize or burn onto the pan. Again, keep the stove exhaust fan running to blow out any fumes from the burning lard. Cool the pan to room temperature.
Wipe out the pan with a paper towel to get rid of any blackened soot. Make sure to wipe all the surfaces, not just the inside, or you may find the pan leaving black smudges everywhere you set it. Use the pan as normal, cleaning it up with regular dish soap, if necessary and always drying it or hanging it up to dry to prevent it from rusting. After a few weeks of cooking, your pan will develop a nice smooth almost non-stick surface and will turn black and smooth.