How to sew slip stitch
Working on the fold in the fabric, secure the thread with a double stitch.
Pick up two threads of the fabric with the needle tip, then slip the needle through the fold of the hem. Bring the needle out 0.5cm along, then pick up two more threads of fabric before returning through the fold of the hem.
Pull the thread lightly as you work to tighten the stitches, being careful not to distort the fabric which makes the stitches visible.