How do I attach shoulder seams that have been bound off???
Your best bet for a nice shoulder seam is to use mattress stitch, picking up a stitch from each side of the sweater.
There are instructions here for 'horizontal seams.' (Has actual pics of shoulder seams.)
Otherwise, you could unpick the BO edges and rework them in 3-ndl BO which is easier, IMO, and often times looks better.
How you do it also depends on what kind of shoulders you have. Are they sloped or just straight across? If they're sloped, it's a bit trickier. I actually do short rows now instead of the graduated bindoff. Then I Kitchner or 3 needle BO.