Use a single or double thread that matches your fabric color. Start by pulling the needle through the inside of one folded edge to hide the initial knot, according to the StitchFloral Tutorial .
It works by creating parallel "rungs" of thread across the opening, which, when pulled taut, disappear. How to Sew an Invisible Secret Seam Secret Seam
If the stitches are visible, the tension is too loose; if the fabric puckers, it is too tight. Use a single or double thread that matches your fabric color