There are many sites that have valuable information for crocheters that aren't just patterns.
The Craft Yarn Council provides information on the standardization of yarns and threads: https://www.craftyarncouncil.com/ Some of the valuable information they provide are
Basic Stitches
Half Double (hd)
Turn chain is 2. Sometimes it counts as the first stitch, sometimes it doesn't. Important to double check pattern for specifics.
Yarn over, insert hook into designated stitch.
Yarn over and pull through the stitch (3 loops on hook*
Yarn over and pull through all loops on the hook.
*Tip: hook should be horizontal, parallel to work, not with the tip ointed downward. Draw a picture to remind yourself...
Herringbone Half Double
Yarn over, insert hook into designated stitch.
Yarn over, pull through the stitch AND the first loop on the hook (2 loops on the hook).
Yarn over and pull through all loops on the hook.
Resources Online
https://easycrochet.com/herringbone-half-double-crochet-video/
https://www.youtube.com/watch?v=kMy1KXdXbMU
https://www.youtube.com/watch?v=1YwnuYrQDLw
https://www.mooglyblog.com/herringbone-half-double-crochet
This is labeled a herringbone single, but it is a variation of the half double: