Toggle active state by clicking the extention icon. Then click on the DOM element and you get JS prompt with XPATH expression in it.
XPATH Builder is extension for building XPATH expressions as the name suggest. Here is how it works:
1) You install it
2) Go on the page you want to build expression for
2) Refresh the page and you will see small batch text in red of the bottom of the extension icon: "0"
3) Now click on the icon to activate it and you will see it is changed to "1" in green
4) Click on the DOM element you want to build expression for and you will get it as JS prompt
5) Copy it, modify it in the way you want (if you want) and paste it
6) Turn off the extension by clicking back on its icon and you are done
NOTE: Do not forget to switch off your extension when you do not need it anymore