Rebecca, understand your question better now. When I braid multiple pieces on a single warp, I generally braid sections that are a bit longer than what I need so that I can cut in between the sections without running the risk of having pieces a bit shorter. To make this a bit easier, I would braid with some type of junk yarn together with the working bundles. I would do that for a few picks to give you room to cut. When you want a flat end, measure the desired length along the selvedge, then add the picks that you will use to separate pieces. There are several ways to finish the braid depending on how you are going to use it. The simplest way to achieve a straight edge is to seal the braid from selvedge to selvedge.
If you can find Rodrick Owen’s book, it is worth getting it: he has a number of starting and finishing techniques that you may like. Saturday, depending on how much time I have in the Q&A I can demo a simple decorative finish