A quick look at standard half double crochet decreases worked through both loops of the first two stitches of a row as well as the last two stitches of the row. I use a turning chain of 1 here to give me enough height to work the stitches, but there are many variations on how you may work a decrease at the beginning of a row and even how you may work the decrease itself, so always be sure to check your pattern!
Single crochet decrease: https://youtu.be/q9CKjHe9008
Double crochet decrease: https://youtu.be/cVoHLVCGGV0
