So, you've bound your neckline "in the flat" and now one shoulder seam is unfinished. What to do? (Don't cut those binding ends off just yet!)
I serge my bound neckline shoulder seams like this:
1. Raise the serger knife to its highest position.
2. Before you put the pieces under the knife, slide the bottom layer back a smidge. The machine's differential feed will grab this bottom layer first and that slight grab will put it even with the top layer for the first stitch. (At least on my serger!)
3. Lift the front of the serger foot (with your fingertips, not with the lift lever) and put the fabric all the way under and against the knife.
4. Release the tip of the foot back on top of the fabric, which is now in position against the knife. (You see a ripple here because I was juggling fabric with one hand and the camera with the other!)
Grab the binding tails and gently guide them (and your shoulder seam) toward the knife as you step on the pedal and slowly start serging over "the lump."
5. Press the seam toward the back and, Voila!
6. After running the serger thread tail back through the stitching, sometimes a stitch or two will show at the neckline edge. When this happens, I get out my permanent fabric markers and color those threads to match or blend. I won't tell if you won't. ;-)