I buy a pack of shmaltz in the store (the white chunks of fat people pull off the chicken before cooking).
I dump it into a frying pan on med/low heat, and put in a bunch of chopped onions for flavor. Then I let it cook slowly until it all melts and the onions get fried.
Then I strain the clear/yellowish liquid (through a mesh strainer) into a container and put it in the fridge. The hardened bits are gribenes, and they are very flavorful albeit not terribly healthy! The liquid hardens into white fat that I use for cooking.