You'll need to watch the video to figure out how to use it.
The tool itself is pretty informative but it's not perfect. There's a bit of give so you won't get extremely precise angles. Also, even though it constrains the extrusions to 90 degrees on the x-y plane, you need 3 hands to hold each extrusion tightly against the tool. If you don't, you'll find after tightening the screws that one of the three extrusions will not be flush with the others and you will need to do it again.
As a sidenote, try to get one of these at the start if you plan to use one. It's kind of a pain to take the Z-drives off if you want to realign the frame after the entire has been built. Not impossible, but wires, skirts, and other things get in the way.
