There are several different ways to end and start a weft. You can tuck ends in on opposite selvedges. You can leave the ends hanging out on the edges and cut them or sew them in after the piece has been washed. Tuck ends in on the same side. I usually overlap the end an inch or two on the edge. If I have joined a new thread on the left, I try to remember to join the next new thread on the right side.