The half-double crochet stitch isn’t used too often in amigurumi, but every now and then I’ll bust it out if I need a slightly taller stitch. It’s very similar to a single crochet, just with an extra step.
Before inserting your hook into the next stitch, yarn over.
Now insert your hook into the next stitch.
Yarn over and pull through.
You should have 3 loops on your hook. Yarn over and pull through all 3.