Well, what you need is a chunk of meat of some type with bone in it. You spice it nicely with some salt and whatever herbs or garlic or stuff you like. And then you cook it slowly for a long time so that you end up with a broth. This is very much like when you make soup. You usually have to add water to get broth, and adding water works better if you are also going to eat your meat because otherwise it ends up really dry and grainy. If you want a clear aspic you don't stir it or let anything make it get mixed up so that the albumin and the fat stays on the top - if you are simmering it you will see scum float to the top of your broth but you can skim it off. If you boil it hard the liquid will mix on its own so it will get cloudy. The low temperature is important if you want it pretty looking. If you have put your chunk of meat and bone in a roasting pan in the oven you will have to wait until it is done. It's perfectly okay to eat the albumin if it does all get mixed in. It's just not as pretty. Albumin is the same protein you get in egg whites.
You need to cook it for long enough that the liquid gets reduced as well as turning a nice colour. Then you take it off the heat or out of the oven and you do nothing at all with it until it is cool at which point the fat will have risen to the top and so will any albumin and stuff like that, so you can take the top layer off if you want to.
If you do a beef roast with just enough liquid and just enough fat the liquid becomes two layers, the brown clear jelly below and the whiter fat above. This is called dripping. You store it in the fridge and when you are hungry you spread it on bread, preferably a robust sort of bread like a cottage loaf, not something fluffy. Bread and dripping is traditional. However mostly now the roasts don't have enough fat on them to make the dripping, so if you want to make bread and dripping you have to put fat in with the roast. This is not a bad thing, because it works perfectly well with a healthy type of margarine. You get beef flavoured fat which is really a soft vegetable oil.
The jelly is nice stuff. It tastes fine cold, just the same way that slices of roast taste fine when they are cold.
To make jellied tongue you need to get the bones with the tongue and that is usually hard to do, but the skin on the tongue might be enough to make jelly and you can always put in a shank bone with the tongue so as to make sure or a spoonful of granulated gelatin. Once the tongue is well cooked you cool it so you can skin it, and then you chop it up into chunks and cook it together with the broth some more and finally you put it into a loaf pan and cool it and if you have done it right the jelly will be sturdy enough that you can slice it. If you heat up the jelly it turns into liquid and when you cool it again it turns solid.
If you don't want your jelly tasting like meat because you are going to put fruit and sugar into it, it is much harder. Then you have to take every scrap of the meat off the bones and the tissue. You usually try taking an animals foot, with the bones and the sinews and the skin and then you are going to have to strain it, using a fine sort of cloth. I've never done this.