How do you warp Supplementary warp on a backstrap loom?

Does anyone know how to warp suplementary warp? It seems like it should be simple but I can't figure out how to do it. Do you add the supplemental warp while you warp or after; do the extra warps have their own heddles; how do they stay in order? It eludes me and there is no imformation on it at all. If anyone can help I would greatly appreciate it, thank you.