More on Flatlocking! this is from an instruction book I purchased way back in 1993.
1) First set up the machine for a Balanced 3 thread stitch (either left or right needle) Wide width, short to medium length stitch.
2) loosen the needle tension almost completely
3) Tighten the lower looper until the thread forms a straight line on the edge of the fabric. The upper looper usually won't need adjusting
On the underside of the fabric The needle thread should look like a "V" intersecting with the lower looper thread
4) Always guide the fabric so that the stitches hang halfway off the edge of the fabric. That way when you open the seam apart there's room for the 2 layers to lay flat against one another