Choose which buttons you'd like to use in your button necklace by laying them out on your work surface until you're happy with the color scheme.
Measure your twine or fishing line, making sure to leave an extra few inches to allow for the clasp and possible back-threading to reposition any buttons that aren't lying right.
Affix one end of your clasp securely to the end of your twine. Make sure it's completely secure.
Decide how you'll string them. You may want to go for a "stacked" look, or you might alternate ways of threading the buttons so that they lie differently.
String your buttons.
Secure the other end of your clasp. Make sure it hangs at the length you intended.