When sewing with pile fabric, whether the pile fabrics are long or short, they must be trimmed n(or shaved) form the seam allowance layers either before or after the seam is stitched. When the pile fibers are extra long, it is usually easier to trim the excess pile after the seam is stitched. When the pile fibers are short, it ay be easier to trim them from the seam allowance area prior to aligning and stitching the seam. Trimming before the seam is stitched will allow for a smoother stitching process since the seam allowance layers are flatter and smoother without the excess pile.
Start by marking the seamline directly on the pile-side of the fabric using a fabric pencil or tailor chalk by measuring the seam allowance form the fabric edge inward. Carefully trim the excess pile fibers using your fabric scissors staying only within the seam allowance and not crossing over the seamline into the garment surface. As a result, you should have a pile-free seam allowance area ending right along the seamline which double up as your stitch line.
Once the seam stitched and finished the seam allowance should lay flat on the garment's wrong side, contributing no significant thickness or bulk to finished seam. This is especially important if the seam allowance layers are ironed toward a plain fabric that makes the seam allowance outline more visible from the garment's right side.
As noted above, high pile fabrics are more comfortable to trim after the seam has be stitched- the pile fibers are easier to control and the excess is faster to trim. However, before the excess pile is removed it is important to take this important step:
Pull any pile fibers that are caught by the seam stitch toward the right side of the seam. The easiest way to do this is by using a pin or thicker needle.