I found a link for DIY GELATIN at maantistaaste.com. Yes, that's the gelatin you could use to make jello.
Originally it was for pork bones, but I used it for ox tail and it worked GREAT. I'd check out the link above to see what they have to say about gelatin, as it's interesting, but let me tell you the basics here.
1. To make gelatin, you need gelatin rich parts. I know that oxtail is gelatin rich. They named skin, trotters (feet), and tails for a pig, but I don't know if this actually applies to cattle as well.
2. Start off like you are making a stock, except leaving out the part where you'd roast the bones. In other words, I just put the oxtail in a big pot, covered it with a lot of water, and heated it. Then, I scooped out the tail and dumped the water as soon as it had boiled for a minute or so, and started the process again with new water. This removes some of the blood and that foamy scum, so it won't be in your gelatin.
3. The original technique used a pressure cooker for 4 hours. I boiled mine on med-low for 12 hours on the stove top, instead, but with the pot covered.
4. Then I turned off the heat (if using a pressure cooker, according to maantistaaste, "remove from heat and allow the pressure to drop in it’s own time. When it’s safe to remove the lid, do so and allow the gelatin to cool a bit before straining.")
5. Then I strained the gelatin through two layers of cheesecloth on top of a sieve. They used a chinoise. This gets rid of any little meaty bits in the broth. Next, they recommend that the broth is "reduced the gelatin by 30-50 percent, then allow[ed] to cool to room temperature."
6. Then refrigerate overnight, scoop off the fat(tallow) that's formed on top and save it for later use in something. Make sure you get the fat bits scooped off the top of the gelatin, and then...that's it! You've got gelatin! If it's not firm enough, you can always pop it back in a pan and reduce it some more. (I made homemade jello with this stuff, and it turned out awesome!)
This is where I stopped my recipe, and have used the gelatin in this form, although it requires some fiddling as recipes don't ASK for it in this form. However, the original website discusses how to dry the gelatin to use as sheets. This can only happen if the temp is above 70 F, because otherwise the gelatin goes liquid and doesn't dry as well.
How to dry it:
First, put it into loaf pans when you are setting it in the fridge (you can heat it up after the fat is skimmed off, and then pour it into loaf pans, if you need to). Once it's fully set, turn the loaf pans out on a cutting board. Slice the gelatin into sheets the way you would bread. Thinner is better - they recommend 3-5 mm, if you can. Then, lay the slices on clean dehydrator sheets - these should dry at room temp, in the air, but NOT with the dehydrator turned on. Again, it will liquify at 70 F, so heating it up will just have it melt through the mesh.
So just let them sit there, dehydrating - you can set up a fan to blow at it, if needed. This can take a few days. Once it's completely dry, it will keep indefinitely in an airtight container.
To use, you would weigh out the amount of gelatin you need and bloom it in cold water for 10-15 minutes. If, like me, you have no freaking idea what that means, you're basically reconstituting this stuff, like you would dried mushrooms. So you sit it in the water, let it soak up a little, and then wring out the extra water and add the now-wet gelatin to your food of choice.