This pattern is a little tricky to follow it seems to me. I went and made a miniture bootie and the straps to see what was going on. The pattern says on row 24 to k10, BO21 and K10. Then on row 25 you are turning right from the end of your last knit 10 and row 25 starts off with knit those 10 again and then CO the 10. Use the yarn you are knitting with as the one strand for the long tail cast on and introduce another strand for the thumb part. (this felt weird but I managed it) I didn't join it in any way really, I just held the new yarn up where I could use it to cast on. After you have the cast on done you can cut that new strand of yarn to 8" and knit the rows and BO with the orginal yarn.
You want to end up with the two straps coming off pointing out into the open area where you did the 21 bound off stitches. That first strap is the right one as the bootie is worn. Now you do the left one.
You already have the 10 stitches on the needle. You need to add the new CO stitches to the right, off to the side where the opening from the BO 21 is. I just used a new yarn and using a new needle made a slip knot and cast on 9 more for a total of 10. It says something about yarn there at the beginning of the left strap, like you already have a yarn there. I don't think I had one. I cast them on a new needle and then slipped those over to the right of the ones already on the (left) needle and then I was ready to do like it says and K10. (The yarn I used to cast on was there to knit with on the next stitches.) Again I didn't join them together, they were just sitting next to each other but when you do the next row and knit across all of the 20 they become joined.
I don't know if this is what the gal intended or if I did it right, but it worked. Maybe Sophie will have a better plan. Good luck.