Sorry I'm just getting back to you on this. Big Sioux is exactly right. I get the fat from around the kidneys. It's a real dense fat. I cube it up and remove any membranes possible and toss it in a slow cooker on low. Stir occasionally and let it render down until you just have oil. I skim any membrane still floating and run the rest through a sieve bag or fruit bag for pressing grapes etc to filter the rest. In a separate pot I add roughly 10% cod liver oil, 30% of neatsfoot oil to 60% tallow and toss in a handful of beeswax pellets and let it all melt down and mix well. I actually did the 2/3rds to 1/3rd tallow to neatsfoot and it was to hard to use. I have also found mixing in jars or plastic containers set in a pot of water on the stove aids in keeping things cleaner. Basically a double cooker.
It's a curriers grease and anything that will be exposed to the weather I treet with this.