Here's a tutorial for how to do binding in the flat.
Here my tank top front and back pieces with one shoulder seam sewn together. (when you're sewing in the flat you only sew one seam together before attaching the binding)
Take your binding strip and lay it right sides together on top of the neckline for the shirt.
I leave my binding strip overlapping a little bit so that it's easier for my coverstitch machine to grab it later on.
(Obviously the whole shirt isn't done, but the neck binding is.)
If you're binding the arms (like I will be because this is a tank top) then apply the binding to the arm holes now, coverstitch it down, and serge up the side seams.