To form a chain into a ring, the ends of the chain are joined with a slip stitch. The slip stitch can also be used to "edge" a row (for shaping or strengthening) without adding height.
As always, there will be a loop on the hook. Insert hook into a stitch. Place yarn over hook and draw yarn through both the stitch and the loop on hook. This creates a new loop on hook so you can continue on to the next stitch, if desired, or can begin a row or round of a different stitch.
Need More Help? Need other step-by-step instructions?
How to work a slip stitch - "sl st"
When instructions call for sl st, insert hook into next st, y o, draw through st and loop on hook.
Source: Crochet Primer, Bernhard Ulmann Volume 64