First, rinse them thoroughly and pat dry. You can cut any membrane from the surfaces (if you like) but it isn't necessary. Some folks like the snap of gristle in the bite. Sprinkle lightly with salt and pepper and fry, bake, broil (you can boil them as part of a stock if you like) or boil them and then run them through a meat grinder to use as part of a dressing/stuffing.
I sometimes just season them and microwave, turning once, as a lunch treat.
If you run them through a meat grinder you can mix them with a dressing (mayonnaise, a touch of mustard perhaps, etc.) and use as a sandwich spread.
Note that livers cook slightly faster than hearts, hearts cook slightly faster than gizzards.