How you use a nursing cover depends upon the style you choose. If you opt for an apron style cover, you'll want to drape it over your head, as you would with a regular apron, and feed your baby from underneath. You'll still be able to see them with some of these styles, as the wire necklines are rigid.
If you purchase one that's tube-shaped, you'll still slide it over your head, but your arms will likely be inside (or at least one of them will). You may not be able to see your baby well, so if they're already having trouble latching, you may want to opt for a different style of nursing cover.
If you go for a wearable cover, like the Storq Cardigan, you may use a fasten or snap to hold the fabric closed as you nurse. Other wearables, like the Bamboobies poncho in this list, will allow you to slide your baby under the cover during feedings.