Not going to lie. When I first saw the need for a half double crochet stitch on a pattern I decided to just use a double crochet. It didn't exactly work out.
Good news, this is an easy stitch and if you alraady know how to double crochet, it's in the bag. For those of you who don't need the pictorial, I'm not going to have you sifting through this post trying to find a quick answer... The only thing that is different between the double crochet and the half double is the fact that you take off all three loops on the first pass. That is seriously it.