How to Increase in Crochet
To increase the width of a basic crochet fabric, 2 or more stitches must be worked into 1 stitch at the point specified in the project instructions.
Single crochet, half double crochet, double crochet, and longer stitches are all increased in the same manner.
There is no specific abbreviation for a crochet increase. Patterns will usually give instructions similar to "2 sc in the next sc", to indicate you will work two single crochet stitches in the next single crochet stitch, and thereby increase.
Single Crochet Increase
Work a single crochet into the specified stitch.
Work a second single crochet into the same stitch.
Double Crochet Increase
Work a double crochet increase in the same way you work a single crochet increase.