Flat felled seams are most commonly used on Jeans and heavier fabrics as it's a really strong seam finish. I have employed the technique for the project I'm working on which is using a light/mid weight denim fabric and doesn't really require a super strong seam, but I like the effect of the visible stitches as a design feature.
Flat felled seams can seem a bit tricky but are really quite simple to do.
First sew your two pieces of fabric RIGHT sides together with a 5/8" seam. Press open and trim one side down to 1/4".
Take the other side and fold in half so the raw edge is meeting the seam then fold again over the trimmed side so that all raw edges are enclosed, stitch close to the folded edge and press.
Voila! you have a flat fell seam.