How do you use the Garrysmod Fin tool add-on?

Answer 1

Create a contraption you want, for example make a plane with wings.

You use the fin tool to make props act like wings, so a flat planel will like to fly straight, adjust the settings in the menu and apply it to the prop you want to make fly.

On a plane you'll only need to make the wings fly because they should support the aircraft itself, though sometimes you can make the whole contraption work with the fin tool (e.g. click on all of the props) to make the plane realistic.