A flat felled seam is quite strong and sturdy, due to the two rows of stitching. It is commonly seen on sportswear and menswear. Take a look at the inseam of a pair of jeans, most likely, they are flat felled.
sew seam, Wrong sides together, with a standard seam allowance (this one is 5/8"). press seam open, trim off one side.
press untrimmed seam allowance in half.
bring untrimmed, folded seam allowance over trimmed seam allowance, and edgestitch.
this is the finished flat felled seam. the other side has one row of stitching, about 1/4" from seam.