I have been working on a cardigan and I have done all three panels and It's a mock-neck. So, I've shaped the back panel, right and left panel's neck line. But to make the mock neck, the parts I don't understand are where it says:
"...starting and ending at front opening edges"
I'm assuming that, since it's a cardigan, it means where the collar will be zipping up in the front.
"KNIT UP 24 sts up right side of neck, 24 sts from back, then 24 down left side of neck..."
What does "knit up" mean?
I think it is an odd way of saying pick up and knit 24 stitches form the right front neck. Starting where the collar will be zipping, you need to convert rows to 24 stitches in the neck shaping. I find this easier to do using a crochet hook to pick up a loop and put it on the knitting needle. Then do the 24 back neck stitches and repeat the converting rows to 24 stitches on the left front.
Hope this helps.