Slip Stitch
Insert the hook in the next stitch.
Yarn over and pull up a loop through all loop on hook.
You complete one slip stitch.
Slip stitches do not add height
and is usually used to move across stitches without add height or to join your work when crochet in the round.
Work in rounds
There are three ways to start your rounds:
Form a ring of chains
Make 4 chains and join them with a slip stitch in the forth chain from hook to form a ring.
Chain 3 (count as double crochet). and work 1 double crochet in the ring.
Make in the ring, as many double crochet as your pattern require.
Join the round with a slip stitch in your first double crochet.
Work in the first chain
Chain 4 (count as 1 dc). Work 1 double crochet in the 4rth chain from hook.
Make in the same chain, as many double crochet as your pattern require.
Join the round with a slip stitch in your first double crochet.
Magic ring
Form a circle with the yarn tail to the back side.
Insert your hook in the circle and grab the working yarn.
Pull up a loop through the ring.
Chain 3 (count as 1 double crochet)
Make in the ring, as many double crochet as your pattern require.
Pull the yarn tail to close the ring. Join the round with a slip stitch in your first double crochet.
Of course you can use any stitch (single crochet, half double crochet) instead of the double crochet.