To make 1 button, you will need a 13mm Roman Blind Ring, dk yarn of your choice and a 2mm crochet hook.
With the yarn you are using, make a slip ring on the hook
Treating the ring as an existing chain or stitch space, make a dc into the ring – as in the following  4 steps.

Dc into the ring again.
Continue around the ring…
  … until you get back to the beginning (about 10 stitches). 
Tie both ends of thread together to join the circle.

Weave one strand of the remaining yarn in and out of the outer stiches. Draw up the thread. As you do this the stitches will pull together to what will be the back of the button. pull as tight as possible and tie to secure. This will form, a shank at the back of the button.

Tadaahh!.. and there you have your own perfectly matching (or not, depending on what yarn you decide to use) button. You can use the remainder of the two threads to attach the button to the finished item.

Nice and simple, and pretty quick to do.
And looks great.