It's pretty easy, you can buy a flicker box like those above or do it the dirty way. I've taped some photoflood bulbs and fixtures together, or have hid a couple of inkies out of shot and have had everything hooked up to dimmers, and gelled to slightly different values of CTO. Have someone play with the knobs while shooting, and you get a pretty good effect.
Point the 1K into the reflector and have someone slowly twist the reflector left to right while strumming the back of it in a circle. It'l bounce and scatter the light in a bunch of different ways.
Add more fill if you feel the need to do so. Go to 1:05 in this video to see Geoff.R being the flame master.
No problem, hutch. That was the first time I had to do campfire flicker, and I actually had no idea how to approach it but we ended up figuring it out. Now that's how I do all of my flicker setups.