The instructions are a little confusing, but it sounds like they're telling you to do a 3-needle bindoff where you are, essentially, knitting the shoulders together in a seam.
If that is, indeed, what you're being told, then there is a video on this sight that will show you how to do this. Click on the Videos tab above, click on Casting off/Binding off and then on Three Needle Bind Off.
Hope that helps! Good Luck!