To decrease in double crochet, work the first double crochet until only 2 loops are on the hook. 
Yarn over, place hook in next stitch, yarn over, pull through stitch, yarn over, pull through 2 loops (3 loops on hook). 
Yarn over, pull yarn through all 3 loops for a completed decrease. 
© Reprinted with permission from Craft Yarn Council of America's Certified Instructors Program