Cut a piece of ribbon the length around your wrist and add an inch or so, to enable the ribbon to be button loosely. (I blu-tacked that bit of ribbon around my wrist because I din't have enough hands to hold the ribbon and take the photo(: )
Cut a V shape in one end of the ribbon. You can make this even by folding the ribbon in half, and chopping off the corner. It may be better to practice this on spare ribbon just incase you go wrong. (I did the first few times!)
At the opposite end to the V shape, sew on your button.
Then, back at the V end, cut a little slit an inch or so from the end of the ribbon to form the hole for the button. When done up, it should sit nicely around your wrist.
Then voila! A nice, pretty, yet simple bracelet.