Increasing Herringbone Stitch

I am a very avid crocheter but I am very new to knitting. I have embarked on a huntress cowl project, before i knew what i was getting myself into. I love the horizontal herringbone stitch it uses but i am at a loss for how to add stitches. The pattern says to add but not how. Does anyone have advice for how to add 1 stitch at the beginning of the row? I also don’t know what it means to “pick” a stitch.
This is the video i used to herringbone stitch.


What’s the name of the pattern? Can you provide a link? (Don’t post the whole pattern) There seems to be several huntress cowls.

This was the shop i bought it from. The section i have having problem with is the slopping part the chest under the arm.