This method works for duck fat yes, but also goose fat or chicken or any other poultry fat. The leftover cracklins are fantastic served as Mexican chicharron en salsa verde. This recipe makes about a pint of duck fat.
Chop the duck fat into little pieces with kitchen shears. If you are working with the tails, snip away the oil gland. It's OK to have bones here and there, but if you are going to later eat the cracklins', you will want to remove any bones.
Put the pieces in a pot and cover with water by just a little. Turn the heat to high, stir well and boil this. As the water boils away, you will want to reduce the heat so that ultimately, the pieces are frying in their own fat.
Later, that fat will turn from milky to clear, and the bubbling will slow down. Strain the fat off the cracklins. Pour the clean fat into a Mason jar and refrigerate for up to 6 months, or freeze indefinitely.