Start by adding your garnish to the tray. If you are using the reduced Jello, more power to you. If you are using the fruit roll-ups, start with a strip from ear to ear and from forehead to the back. Build up a coverage over the entire skull - this gives the first layer of meat a bit of a grip and, should they get to it, a bit more gore at the end.
This is the skull completely covered. If your roll-ups are fresh, this should be easy. You may moisten your fingers to help with the joining if needed (use a glass of water). I have also added the olives in the open tomatoes for the eyes. You can also use yolks from boiled eggs or boiled eggs themselves.
I usually start with a layer of ham on the inside as it has a nice fleshy color. It may not be kosher - but I don't think a head of meat should be. You may need to cut some of the slices into smaller pieces as needed for the nose and fold pieces for the lips.
Depending on the look you are going for, you can also other meats for additional layers. We usually use turkey or chicken, but you can also use roast beef or even corned beef for a nice burned look. Ham itself leaves a nice flayed look.
In this case we used roast beef for hair and eyebrows. As a final garnish, we sprinkled a bit of rice on the head to simulate maggots.
Wrap the head in saran wrap and refrigerate until ready to serve. Have bread and condiments as needed, as well as forks and plates.