Andouilles are usually prepared raw and then cooked in lightly salted water for the length of time indicated by their size. After this they are grilled.
Andouillettes, on the other hand, are bought already cooked. They only require a few shallow incisions to be made on each side, then to be wrapped in paper, buttered or greased with lard and finally gently grilled.
For either the Andouille or Andouillette, the usual garnish is mashed potato.