I made this nursing cover almost entirely during a naptime, including this blog post. Like I said: ridiculously easy!
Muslin blanket (from our stash of, like, 400 new blankets)
2 yards of wide ribbon (the only thing I bought for this)
I started by folding down one corner of the blanket and pinning the ribbon to it - I pinned from the back to make sure the blanket was straight against the ribbon.
After I did that and got my straight fold, I cut the long end off the ribbon (about a yard, maybe more - nothing about this is exact) to make a strap, unpinned one end, and sewed on the end of the strap so it would be under the edging ribbon.
Then I sewed along all the edges of the edging ribbon, turning the ends under, trimmed the strap to the length I want it, hemmed the end, added a big button to the blanket and buttonhole to the strap*, and trimmed off the folded-under corner. Easy peasy!
*Not shown, because I'm going to ask my mom to do it - I don't know how to do buttonholes. But you could also do just a decorative button with a snap on the back side and strap if you wanted.