Begin by sewing a Plain Seam.
Fold over and under both edges of the seam allowance 1/4 inch. No raw edges will be showing.
(for some reason I can’t find the pictures I *swear* I took of the next step!)
Stitch edge of the fold through all layers of fabric.
The underside will look like this
And the top, showing side like this
A lot like the Welt Seam on this side.
The Book says: “A Flat-felled Seam provides a neat attractive finish to both sides of the garment. This type of seam is usually used for sport clothes and reversible garments.”