Joe, there is a good pictorial
here. I have also used this method with KOH glycerin, and it works well but it takes me a while to build up a collection of milk cartons!
I clean the milk cartons out, open the tops up, fill with sawdust, then pack it down with a stubbie. Then when I have a box full of cartons I pour the glycerin in. It takes a while to soak through so I keep topping up until they are full. I then chuck them on a roaring fire and they chuck out lots of heat.
Oh and don't pre-wash your bio if you are going to burn the glycerin!