Enter a title, description, tags, your API token, and mark as
Private if necessary.
Click Submit Model!
Follow the instructions for a download dialog that appears.
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.