I render my own beef tallow. My butcher saves the fat for me and gives it to me for free. This method can be used for beef, lamb or pork fat.
Method 1 (time consuming but no horrible boiling fat smell):
1. Cut the fat in to pieces (about 1 x 2" is best)
2. Fry slowly in a large frypan over a low heat. Keep pouring the fat off into a saucepan. You need to sit by it during this process.
3. Once it is all reduced and no more fat is coming out, add twice the quantity of cold water to the fat in the saucepan, bring to coil and simmer for 1 hour.
4. Stick the saucepan straight into the fridge, or do as I do and pour into a stainless steel bowl, then refrigerate for 24 hours.
5. After 24 hours take the bowl/saucepan to the sink, use a sharp knife or skewer to make a couple of holes and invert the bowl to let the water run out.
CAUTION: The fat may also pop out and water will splatter EVERYWHERE. Be careful and if you want you can use your hand to hold the fat in the bowl while that water comes out.
6. Pop the disc of fat onto a plate, scrape any jelly like substance off the bottom (if you've done step 2 properly you hardly ever get any) and store in a container in the fridge.
Method 2 - easier but much smellier. If you can do this on an outside stove/fire your house and loved ones will thank you for it.
2. Add the pieces of fat to a large saucepan.
3. Bring to boil and simmer for about 4 hours, skimming the gunk that rises to the top off about every 15 minutes.
After 4 hours, follow steps 4-6 from Method 1.
You now have a nice quantity of lovely clean tallow to cook with, and you know it has no impurities or additives.