Under Prepare Maps, toggle Painting, Normal, and AO maps as appropriate, build UVs, then Restore Visibility, then Bake. These steps may take some time, depending on complexity of the textures and the number of subtools in your model.
Under Decimate, run Preprocess followed by the Decimate function to reduce your polygon count. This can also take quite a while for big models. The default level of decimation is 25%, which you can change with the slider.
Under Upload options, enter a title, description, tags, and mark it as
Draft if necessary.
You can also set a maximum texture map size, which is useful for maintaining good performance if you have a lot of textures.
When the upload is complete, you can view your model on Sketchfab, adjust rendering settings, and save the default view.
For more detailed instructions and other information, visit the blog or watch the video above.