Once you have warped and dressed the loom you are ready to begin weaving. This may be the hem or you jump right into weaving your cartoon and deal with the warps later once off the loom. In this video you will see I am weaving across the warp using several bobbins of the same weft. I use several bobbins so there is less chance of pulling in. You must “bubble the weft as well, to make sure there is enough weft to weave over and under each warp and not pull the selvages (edges) in. I have seen may tapestries where the selvages are fairly straight until the weaver reaches the top of the design or other place where there is one color from selvage to selvage. They make the mistake of using one or two bobbins and that is where they pull in. I have done this too and the solution is several bobbins, one per about every four to six inches or so. The more you tend to pull in, the more bobbins you should use.
Here is a video showing the start of weaving and how I use a bobbin.