There's supposed to be a little piece of square steel stock about two inches long that fits in one of the slots that holds the wire. Its purpose is to keep the wire from getting too hot. Can operate without it, but it helps to keep the heat even. Can probably pick something like that up at the hardware store if yours is missing.
Just need some trial and error in adjusting the burn wire to burn the feather to the quill, but not run into the cresting. I often plug and unplug my burner a half dozen or more times to adjust the wire. When it gets hot, I quickly slide an arrow into the holder and give it a quick look. If it doesn't look right, I pull it out and turn off the burner, adjust the wire and do it again. Doesn't hurt to fletch a sacrificial arrow or two to test to make sure you have everything right.
When burning, be sure the arrow nock is seated all the way in to the nock holder and that you have the shaft snug against the v groove. Turn smoothly, and just fast enough to slice off the feathers. Always keep the shaft moving -- in, out and turning -- or it will scorch the cresting and may light the feathers on fire..