Not hard to do. I think this is the start for your thumb. You need to separate out the thumb stitches, the 12 that you place on your stitch holder. Now you need to cover up the “hole” left by the 12 stitches that you’ll work as the thumb - that’s where you cast on 2 stitches.
To cast on, just wrap the yarn around your thumb and stick the right-hand needle through the loop. Do this a second time, and presto! two stitches cast on.
Gloves and mittens are NOT the easiest things in the world to knit! Congratulations on starting this pattern.
PS - when the directions say Work 1 round even, they mean to just work whatever pattern stitch you’re doing, around all your needles, ending where you started. One round - once around your needles. Then stop. Read directions to see what next.