At first you don't succeed ...
What I did was to take a 2 pound piece of pork loin and and put it in a covered baking dish with 1 cup of (cheap) white wine and a little salt (maybe a quarter of a teaspoon). This was put into a 350°F oven for a bit over 2 hours - the pork was tender enough to tear apart with a fork.
it's really difficult to make a photo of meat jelly visually interesting
Tasting this stuff was an interesting experience. A part of me kept expecting sweet fruit flavors, so completely un-subtle flavor of meat came as a shock with every single bite. I also had trouble reconciling the coolness of the jelly with the taste. The thing is, it wasn't at all an unpleasant experience - it actually tastes quite nice - it was just surprising. I suppose one gets used to it after a while.
The color comes from the pork, and maybe from sugars in the wine caramelizing (is there enough sugar in wine to do that?). Some of the medieval recipes for meat jelly say to make it all sorts of colors using various substances, so there must be some way to make less colorful jelly as a base. Perhaps a different type or cut of meat (medieval recipes suggest things like cow feet or sheep's feet or even veal bones) or maybe a different type of wine or adding vinegar would work better.
Anyway, now that I know I can intentionally make meat jelly, I'll keep experimenting and see how close I can get to one of the actual medieval recipes. I doubt I'll be able to get a lot of people to eat it - even if it is nicely presented, colored yellow or red or blue, and with some pieces of meat mixed in, but it shows up in medieval cookbooks often enough that I just have to know.