I'm going to do a terrible job of explaining this, but here goes...
I use a plain steel rod and cut a stout groove about a 1/4" back from one end. I place this in the bore just below flush with the muzzle with some type of packing around the rod about an inch down. I've used steel wool, Scotch Brite, cotton, masking tape, etc. and it all seems to work about the same. Stand the barrel vertically in a vise, melt some lead in a large spoon and pour it right down the muzzle. This will cast a perfect lap both sized to your bore and well-siezed to the lapping rod. I like about an inch in length for most most applications. A little less for small bores, and a little longer for big bores.
The lap will be stuck at first but a little tapping at the rod end with a small hammer will free it. I drive the lap about two thirds the way out of the muzzle and coat it with diamond compound (fine valve grinding compound will do nicely) and then get started with the back and forth. After awhile, you'll need to recast the lap, recharge it, and repeat the process until you're satisfied with the results. If you're going for a match-grade bore and using progressively finer grits, don't forget to clean everything you're using when changing grit sizes! That means a new lapping rod, fresh lead, wash your hands...