This is so easy to make and is a really great cooking oil. You can either make it on the stove or in the oven. I do mine in the oven usually, after something else has finished baking.
Put unsalted butter into a pan or oven proof dish. Heat the butter until it melts and then simmer gently until the white particles have turned golden brown. Strain through a double layer of cheese cloth or handkerchiefs. It doesn't need refrigerating (but can be if you like or don't use it much). I just keep mine in a jar by the stove and use it for frying most things (especially onions).