How to Crochet: Slip Stitch Seam
Align two pieces, with right sides facing each other, matching stitches to be joined.
Make a slip knot on your yarn. Insert hook through first stitch of both pieces, draw through slip stitch, insert hook through next stitch of both pieces, yarn over, draw yarn through both stitches and loop on hook, in one motion.
Continue across edge to complete seam. Fasten off.