The goal is to put as much Vaseline on the outside of the cotton ball as possible while keeping a "dry" core inside the puff. I found putting a handfull of cotton ball in a Ziplok bag along with a big glob of Vaseline seem to work find. That prevents me from overworking the balls - which would force too much Vaseline inside. I just leave them stored in the Ziplok bag, or if space is tight I transfer them to a snack-sized bag.

When you want to light them, pull the ball apart a bit and apply the spark to the dry core. It usually lights very easily. The Vaseline will make it burn a fairly long time.

I've also been known to smear some of the Vaseline on the adjacent wood tinder under the theory that it would help it light. I'm not sure if it really helps much though.