Flat felled seam

First you seam along the seamline with wrong sides together. Then you press the seam allowance open, and then press them to one side. Then you will trim the edge of the seam allowance to 1/8. Fold the edge of the upper seam allowance under 1/4 and press over the trimmed seam allowance. Then edgestitch along the fold chose to the edge of the fold.