Ah, backstrap weaving, something that is on my to-do list. I think the traditional method is to wind the warp between the front and back rod with a cross in the middle or toward the back. You then tie one rod to a tree or post and the other to yourself. While sitting you insert the shed stick or roll behind the cross and the beater in the front. You then have to tie up a heddle rod, looping it around every other thread for plain weave. The shed stick sits behind the cross. I think I've already used too many words that sound like gobbledy gook, if you are new to backstrap weaving.
I dug around a bit to see if I could refer to you to a good resource. Interweave is disturbing book called Weaving in the Peruvian Highlands that has a few photos of how backstrap looms are set up, and while there is no step-by-step information on warping there sure is a lot of eye candy! I went into the Interweave library--a treat unto itself--and dug around for a book to refer you to and came up empty handed (and fell off a chair while reaching for a prospect on the top shelf; shame on me, I'm a safety monitor). I do know that Weaving, Spinning, and Dyeing by Rachel Brown has a chapter on backstrap weaving, but our copy has wandered off somewhere so I couldn’t look at it to tell you if my description is apt. I bet you could find it at the local library.
Here is a You Tube video that shows a weaver on a backstrap using a rigid heddle for the shed stick: www.youtube.com/watch?v=LbzUdaMjtws. It at least gives you an idea of the goal. I’ve been keeping my eye on Travis Meinlof, a clever fellow in San Francisco. He has designed a cardboard backstrap loom and has posted photos to Flicker with a sort of step-by-step on how he warps it: http://flickr.com/photos/mrtatiana/sets/72157604206361659/.
Me thinks it might be time for an article on backstrap weaving!