Well, theoretically (as I've said, I have not done it yet), you pull the bullet with your Leatherman Juice (very doable). Then you empty out most of the powder ( just how much is the salient point.) You then stuff a piece of rag into the cartridge in place of the bullet. The idea is that you then fire the cartridge from your gun. The small amount of remaining powder left in the cartridge ignites the piece of rag, which is then expelled out of the muzzle of your gun (presumably at a high rate of speed) burning as it goes. You then scramble frantically over to said burning piece of rag, grasp it with the aforementioned Leatherman Juice, and then stick it under your carefully gathered tinder pile. Voila--inferno.
As I've said, I have never tried it. I hope somebody chimes in who has who can correct me if I have the details wrong.