With a precooked zampone, all you need to do is to bring plenty of water to the boil in a pan large enough to hold the zampone itself; when it boils, just place it, still in its alluminum wrapping, in it and cook it for 30 to 40 minutes. Nothing more, nothing less than that. Usually, cooking instructions are printed on the box, so you can always follow those.
If you have a fresh zampone, you have to start prepping it the night before serving, by immersing it in plenty of cold water for the night. By doing so, the zampone is washed of the extra salt used to preserve it, which results in a milder, more delicate taste.
On the day of serving, prick the zampone with a fork and, with a knife, make little cuts under the trotter's nails, so that extra steam can be released during cooking time and the zampone will not "explode."
Once this is done, wrap it in a clean, white tea towel and secure it with kitchen thread at both sides. Place it in a large pan, cover with water and let it boil for about 3 hours on a medium heat.
When cooking zampone, nevermind if fresh or precooked, remember not to add salt to the water, as it is already pretty flavorsome on its own and extra salt would only spoil its taste.