The slip stitch is used when a hardly-noticeable seam is needed. The result is often more elegant than a machine stitch. It is mostly used to close an opening or to sew hems in a more invisible way.
Pin both fabrics along the edge. Choosing a thread that matches your main color, insert a needle in the fold of one fabric and pull it 6 to 8mm away into the same fabric between both layers. Then, insert it back into the other fabric, catching only a thread of fabric each time. Repeat the process, stitching regularly. Once you have finished sewing, knot the thread and place the knot into the folds to hide it. Do not pull the thread too much to prevent the fabric from puckering and having a distorted seam.