Undercoating: tips???

My rocker panels and rear quarter panels are getting chewed up. I don't want them to rust so I was thinking about putting an undercoat like herculiner or Rhino on. How would I do that and Whats the bestbut cheapest undercoat. I'm new to body work and any help would be awesome.

If you want the best protection for under body and something that can be easily touched up if it gets damaged use the truck bed liner stuff. They have two different brands at walmart and can probably pick up the stuff at other places. I am planning on getting some and completely coating the entire underside of my jeep to start with and then go from there if it protects as well as I want.