I use a pig's head without the jowls here. I use the jowls to make jowl bacon or guanciale. This results in somewhere around 2 pounds of pig bits once you cook the head and chop it all up. You can use any selection of pig bits, but you need collagen, so a head or pig's feet are necessary.
Set the pig's head in a large kettle and cover it with water. Bring this to a boil and skim off all the scum that floats to the surface. When this is done, add the carrots, onion, bay leaves, juniper and black peppercorns. Simmer this, adding more water if the level drops below the pig's head, until the meat wants to fall off the bone, about 3 hours.
Carefully remove the pig's head and pick off all the meat and random bits. I toss the eyes and the palate, which aren't very tasty. Chop everything up very fine, and combine with the spice mix in a bowl.
Strain your pig broth and pour about 10 cups into a large pot. Bring this to a simmer and add the cornmeal and buckwheat flour, stirring constantly so you don't get lumps. Add salt to taste. Cook this, stirring often, about 30 minutes.
Add the chopped pig bits in with the mush and stir well to combine. If you happen to be using the unflavored gelatin, now is when you would add it. Let this all cook for 10 minutes or so.
Pour this into loaf pans, or a terrine pan. While not strictly necessary, it will make the scrapple easier to remove if you line the pan with plastic wrap first. Pack it in well. Let it cool to room temperature uncovered, then cover it with plastic wrap and refrigerate overnight before slicing and frying.