There are at least half a dozen ways to prepare your weaving for fringe, but here is my way. At the loom, I weave 1/2 inch of close plain weave with scrap yarn, then around 4 inches of loosely woven plain weave, then another few picks of close plain weave (still in scrap yarn). Then I weave 4 plain weave picks in the "real" weft yarn, then I commence the actual weaving. At the end of the piece, I reverse this sequence. If I'm doing multiple pieces before I cut off, I'll throw a contrasting pick as a guide for cutting apart, then repeat the whole shebang.
When I cut off, I serge (or zig-zag on a conventional machine) along the 1/2 inch bands to secure everything (black thread in this picture), then toss the weaving in the washer & dryer. The areas of loose plain weave keep the fringe from getting tangled in the wash. Then I cut off the serged strip, easily remove the rest of the scrap yarn in sections, and twist, ply, and knot the fringe. Then steam press the whole piece.
I think the fringes look much nicer when the washing is done before the twisting and plying. Just my opinion.