Shaping thumbs

Hello, can someone help me?
I am knitting my sister some mittens. They are 2 stick, 34 stitches.
I have just got to the thumb, and the pattern says knit 23 stitches, turn, then next row says cast on and P3st, P5 and turn. I don’t understand! Do I cst on 3 st then purl them immediately, and how do I do this if the wool is on the other side?
I have only made scarves and hats before!
Thank you x:-P

To cast on to a wip, move the R needle to the L hand and use a knit CO, then purl them and the next 5 sts, then turn again. The yarn will be in the right place, just follow the instructions step by step and it works out.

Thank you, i’ll have a go! X x x