How to Decrease in Crochet
To decrease the width of a basic crocheted fabric, 2 or more stitches are worked together, by leaving the last loop of each stitch on the hook then working them off together. Single crochet, half double crochet, and longer stitches can be decreased this way.
Patterns should tell you what abbreviations are being used for the decreases for that particular pattern. Usually the decrease will tell the type of stitch that is being worked as well as how many stitches are being combined into one.
For example, to decrease by one stitch in single crochet, you will combine two stitches into one in a single crochet two together, or sc2tog. To decrease by two stitches in double crochet, you will combine three stitches into one in a double crochet three together, or dc3tog.