To sew a flat felled seam, sew your fabric with wrong sides together and a seam allowance that is double the width of the finished seam.
Cut one end of the seam allowance down to ¼ inch.
Press the seam to the side with the cut seam allowance.
Fold the intact seam allowance in half so the raw edge is folded under and pin.
Sew the folded seam down 1/8 of an inch from the fold.
And now your flat felled seam is complete.