Raglan sweater pattern

[Sweetie]

Hi folks, new to knitting and trying an easy sweater having had some Chirstmas success with scarves and hats. The pattern is the Lion Brand Wool-Ease Chuncky Adult Raglan Sleeve Pullover. I'm at the point of doing the neck shaping and am confused by the instructions as follows: "Neck Shaping Cont to work Raglan armhole shaping as for back, join 2nd ball of yarn and bind off centre 8 sts. Working both sides at once, dec 1 st...etc. I know how to bind off, but I don't understand the concept of binding off centre 8 sts, where I am supposed to join the 2nd ball of yarn and how I could then work both sides at once if I have bound off stitches in the middle? I'm sure this will be clear once explained, but right now, I'm stuck. Your help would be appreciated.