Sally again here..
I have also recently made two nursing covers. I didn't really follow any tutorials. I just looked and measured my current non boning cover and just made it work. The key for the nursing cover to stay up is to have the straps sewed over the boning wire.I would also recommend doubling the fabric or using a heavier weight fabric. The size is dependent on the person you are making it for. It doesn't take very long to make either! My girlfriend loved the black and white one I made for her! Such a great gift idea and can be fairly inexpensive compared to the hotter hiders sold in babies r us!