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.
Pin with wrong sides together
Sew
Cut one end of the seam allowance down to ¼ inch.
Cut
Press the seam to the side with the cut seam allowance.
Press
Fold the intact seam allowance in half so the raw edge is folded under and pin.
Fold the seam over
Press again.
Sew the folded seam down 1/8 of an inch from the fold.
Sew
And now your flat felled seam is complete.