Mini Tutorial: How To Sew A Flat Felled Seam
1. Align the two corresponding seam edges with wrong sides touching and pin the edges perpendicularly to secure. This initial alignment with fabric wrong sides touching is what makes a flat felled application unique- in a conventional seam application, seam edges are always aligned with face sides touching which is what ensures the seam allowance layers end up on the garment's wrong side.
2. Machine stitch the seam at the designated seam allowance. In this instance the seam allowance width is 1/2".
3. Iron the seam allowance layers open as shown. At this point, the seam allowance layers will correspond to the garment face side. Ironing the seam allowance edges open will make them easier to trim (grade) in the step to follow.
4. Trim one of the seam allowance layer down to about 1/4" or half of the seam allowance. In terms of which of the two layers you should trim, keep in mind that the side that is trimmed will receive the topstitching. It is important to keep this direction in mind if this seam will intersect with other seams or be positioned to visually match other seams (a garment's side seam for example).
5. Fold the untrimmed seam allowance layer in over the shorter, trimmed layer to encase its raw edge.
6. Next, iron the folded seam allowance layers toward the direction of the trimmed edge. Pin (or hand baste, as shown below) the folded edges down in this position to keep them stable during the topstitching process.
7. Topstitch along the open folded edge following it as a guide in the stitching process. For best results, stitch close to the edge maintaining an accurate, even position.
8. Last but not least, remove the hand basting stitch, if any, and give the flat felled application one last press to complete.