Iron spits are made with the Smithing skill by using an iron bar on an anvil (a hammer is required) with a Smithing level of at least 17. This provides 25 Smithing experience. They can be used to cook raw bird meat, raw beast meat, raw chompy, and raw rabbits. You can get the raw food by using the Hunter skill to catch animals, or chompy bird hunting to get chompies. To cook the raw meat, be sure to have at least a Firemaking level of 20 and then use the meat on the iron spit, and then on a fire.
One is needed to make a skewered chompy for the Recipe for Disaster/Freeing Skrach Uglogwee subquest. Raw jubbly cannot be cooked with an iron spit, and instead must be cooked on an ogre spit-roast.