There are two kinds of slipstitches: even and uneven. Use the even stitch to join two folded edges, as in mending a seam or finishing a cushion; use the uneven stitch to sew a folded edge to a flat surface.
For even stitches, bring two folded edges together. Take stitches about 1/4 inch long through one folded edge and then the other. Work from the right side of the fabric.
For uneven slipstitches, fold the edge under ¼ inch; press it. Take a stitch about 1/4 inch long through the fold, then a very short stitch through the single layer.