The SketchUp exporter lets you publish 3D files straight from SketchUp to Sketchfab.
You can install the SketchUp plugin directly from the Extension Warehouse. This is the best way to install the plug-in because it will automatically stay up-to-date.
If you have an older version of SketchUp or prefer to install manually:
Follow this great tutorial from Alex Schreyer to prepare and upload your SketchUp scene.
If you're using SketchUp 2013 or newer, you will also have options to enable or disable Export edges, Export textures, Export two-sided faces, and Use 'color by layer' materials, depending on your needs.
When the upload is complete, you can view your model on Sketchfab, adjust rendering settings, and save the default view.
This video demonstrates the upload process: