When sewing with high pile fabrics, the pile fibers should be trimmed from the surface of the seam allowance layers to eliminate bulk and thickness in the finished seam. When the pile is extra long, the excess is trimmed after the seam is stitched and not before similar to what you would do when sewn with short pile fabrics. Before the excess pile is trimmed off the seam allowance, one important step must be taken:
Using a pin or blunt end of a needle, pull as much of the pile that was caught by the seam stitch to the face of the garment as shown. Doing so will ensure that you are not accidentally trimming pile fibers that belong to the garment's right side. It also creates a fuller look at the seam which hides the seamline underneath for seamless look.
Only after the caught pile has been pulled back to the garment right side can the excess pile fibers be trimmed from the seam allowance layers.
As a result, the seam allowance layers will lay completely flat on the garment right side especially when the seam edges are finished together using an overedge stitch like serging or zigzag (on your home sewing machine). If the high pile fabric is stitched to a plain fabric, similar to the example shown here, the seam allowance should be carefully pressed flat and is directed towrad the plain fabric layer.