There are a multitude of ways to go about making a button necklace, but they all have one thing in common - a lot of buttons. The general rule is, use simple ones for filler and add some wow factor somewhere to draw your eye's attention. You can use string, elastic, wire, or the like, to string them together.
It's a great project to think about when you're not certain whether to throw out that button tin (the answer is "NO", never throw out a button tin!) because you never seem to find the right number of buttons for your cardigan there, but you will always find enough buttons to make a button necklace.