French Knot (Single Wrap)
Bring the needle out through the fabric and holding the thread taut and flat to the fabric with your left thumb. With your right hand twist the needle round the thread.
Still holding the thread firmly take the needle back into the fabric one thread away from where the stitching thread emerges from the fabric and insert the needle.
At this point it is sometimes helpful to brush the knot down the shaft of the needle with the nail of your left thumb so that it is sitting firmly on the fabric. Pull the thread through to the back of the fabric.