Dry Render: Dice or grind up the fat (will melt faster) and add to a covered roaster or a crockpot. Some people leave it in the oven overnite...I would want to watch it. As the fat melts, you ladle it off. Then let that melted fat get hard again. Then do it again but this time add about 1 tsp salt to each pound of fat. This salt attaches to the pieces (blood, small bits of meat, etc). When this fat gets cold, you should be able to cut the fat off the top of the pot, turn it over and scrap the "trash" off. What is left is rendered tallow. I prefer not to do it this way...I have done it once. Mine ended up being yellow like butter, and when I used it in my recipe, I could still smell the tallow. Plus is was messy.
Wet render: Dice or grind fat up, add to pot then add water with salt to cover the chunks. Start boiling... It will take a while. When done, cool. I render in the winter. I put the covered pot out on the porch. Let nature chill it off, not the frig. Next morning, the melted fat will be at the top, cut it out and scrape off. You will have bits in the bottom of the pot, you can feed to the animals or trash. Do this several more times, just make sure that you have quite a bit of water in the pot. Your tallow will be white and very clean. The last thing that I do is add the cleaned fat into a stockpot without water and melt very very slowly...There will be water caught in your cold tallow (it will mess with you tallow weights). By melting slowly, any water in the tallow will sink to the bottom...ladle off pure tallow off the top.
TRex
