I have come to the part where I need to divide body and sleeve. What does “Thread next 50 stitches of sleeve onto spare yarn” and how do I do this?
You are going to be removing the arm stitches on both sides (one at a time) and will finish them later.
Take a piece of scrap yarn long enough to go through all 50 stitches with about 8+inches on either end and thread it through the 50 stitches and take them off the needles. I loosely tie the ends of the scrap yarn so it won’t pull out, but that’s up to you.
At this point the pattern often tells you to cast on X amount of stitches for the underarm. You can do backward loop, cable cast on, or knitted cast on for that. Then you’ll join and knit across the next part of the body and repeat with the other arm. You’ll now be knitting in the round down to the bottom.