Is there anything in documentation ?
1) Add a Navigation node on you scene, add a NavigationMeshinstance as child, below the NavigationMeshinstance there is all your static meshes you need navigation, they must be static boy with a collision node .
2) Select NavigationMeshinstance on hierarchy view, there should be a new button appearing on the top of 3D view named "Bake" , click on it and Godot will generate the navigation.
There is a project example you can download for Godot 3
https://github.com/DevMagicLord/Godot3/tree/master/projects/navmesh
