Many shooters ask about making their own ballistic gelatin for testing bullets. Perhaps the easiest way to get started is with a Gelatin Innovations ordnance/ballistic gelatin kit from Vyse. The kit contains everything you need except water, a bucket and a scale to make three 6 1/4 x 6 1/4 x 16-inch blocks of 10 % gelatin. This is the standard for the FBI ammunition tests. The directions are easy to follow and the blocks are very high quality. If you want to put your favorite load to the true FBI test, make sure you calibrate the blocks using a BB gun as described in the complete instructions.
The below video contains the basic instructions for making a block. As you'll see, it's really not that hard to do. It is a little hard on the wallet, though, as a single gelatin block ends up costing about $30, and they're one-shot deals.