Take your first button, any button will do, preferably with 2 holes. Pass the thread/string(I used waxed cord I had) through the two holes, leaving the ends out the back of the button
Get your first hair clip. Pass the two ends of the string on your button through the hole in your hair clip. If you want you can also glue the button to the hair clip, if it needs to be fixed a certain way up. (I didn't glue)
Tie the string ends in a knot at the back, trim ends if necessary. Now you are done!
Repeat steps 1-3 for your second hair clip and enjoy :-D