The main challenge is getting the center of the sausages up to temperature.
However, you don't want boiling water touching the casings, that will cause them to fall apart. (simmering is okay, however)
In general, you want to get the sausage almost up to temperature, using whatever method you like, and then sear the sausage to give it a nice Maillard exterior.
Different methods for getting it up to temperature:
Cook them in a pan, with 1/4" of water covering them. You'll probably want to put a lid on it steam them at the same time.
Cook them in a pan, with water covering up to the top of the sausages.
Put them inside a heat-resistant cooking bag, and drop it in boiling water. If the sausage has some good spices, this is a great way to cook them, because this avoids leeching some of the spices out.