Unfold top edge of binding.
Leaving 2 cm extra at beginning and end, pin top edge of binding to edge of neckline. Make sure you're pinning the right side of the binding to the wrong side of the neckline so the binding will be seen on the outside of the garment.
(Conversely, if you can't find a nice binding that co-ordinates with your fabric, attach the RS of binding to RS of garment. The end result will be a binding that is on the inside of the garment that can't be seen on the outside!)