Pick out the buttons you want and decide on a pattern if you want one. (Alternate option: pick them randomly from a bag or basket and glue them on as you grab them. This will result in a more random arrangement of the buttons.)
Put a dab of glue on the vase and then put the buttons on the glue. Keep in mind that they will dry fairly quickly.
Once all the buttons are attached and you have covered your vase, you can go back and fill in any parts you may have missed or you don't think look filled enough with smaller buttons.
Add a candle and see how wonderful the candle holder looks shining through the button holes!