My favorite neck covers are the Amigo brand from Horseware. Since they fit close to the blanket, there seems to be less chance of other horses grabbing the neck covers and pulling them off. I also like the neater appearance.
I start with a small piece of 2 inch webbing and a 2 inch rectangular key ring.
Place the neck cover against the blanket and mark with clear tape where you are going to attach the rectangular neck ring. The Amigo neck covers have a velcro attachment that slides through the rectangular neck ring attached to the blanket side.
Place the 2 inch retangular ring inside the webbing and fold into thirds.
Stitch into place either by hand or sewing machine.
Attach webbing/rectangular ring piece onto blanket where it is marked with tape. Sew into place. For heavy duty thread, dental floss can be used.
Instead of opening the velcro closures every time I take off the blanket and neck cover, I found 2 inch wide spring clips that I use to clip the neck cover closed.
If you prefer to use neck covers that attach to the blanket with elastic straps, you can do this by sewing three D rings onto your blanket. I usually place the center ring about 8" from the neck. The two side Ds are usually placed about 15" down from the center D and 8" in from the edge of the blanket. Be sure to test your measurements first so that they fit properly on your blanket.