Supplementary Warping Without a Second Warp Beam
I would like to weave a scarf project that uses a supplementary warp. Under equipment, it says to use ". . . a second warp beam or large plastic bottles for weighting a supplementary warp." Can you give more information for weaving with a supplementary warp if you don't have a second warp beam?
I've gotten quite a few questions on how to deal with projects that recommend two warp beams when you don't have a second beam, so I'll make this answer a bit more general than your question.
Usually, you need a second warp beam because you have two different tension requirements for the warp. Often, one warp is a supplementary pattern warp (as in turned supplementary-weft structures) in which the two warps are threaded alternately (as many threads in the ground warp as in the supplementary warp). These are the most difficult to deal with if you don't have two beams. If you are able to wind both warps on the warp beam under high and even tension, my method of choice is to slide a strong, smooth dowel under the pattern warp, over the back beam, and down to hang under the warp beam. I then suspend fairly heavy weights on both ends of the rod (sometimes one also in the center), enough weight so that the tension on the pattern warp is equal to the tension on the ground warp (I have 2 lb weights, but bottles filled with water will also work). The key to success with this method is high and even tension on the warp beam. Without that, you'll have all kinds of tension difficulties as you weave. If you wanted to do many projects like this, I'd spring for a second beam.
When there are fewer threads in one of the warps (as in backed cloths or doubleweaves in which one weave has a much more open sett) and/or one warp appears in spaced stripes (as with a supplementary pattern warp in the borders only or narrow seersucker stripes, etc.) I prefer not to beam both warps. I wind the main warp in one chain and beam it. Then I wind separate chains of the striped/less dense warp (each stripe as a chain if that works; otherwise I divide the total number of threads into several chains). I place a choke in each of these chains in a position that allows me to secure it to the back beam for threading. Then I thread both warps. I tie both warps to the front apron rod and then undo the chokes and suspend weights from each of the chains to hang below the back beam. These weights can be secured in several ways. I like to make a lark's head knot of the chain, place an S-hook in the loop, and hang the weight from the S-hook. The remaining yarn in the chain rests on the floor and I usually keep each chain protected from lint by putting it in a plastic bag. Advancing this warp is easy: you remove the S-hook, pull out the lark's head knot, make a new lark's head knot, and reinsert the S-hook.