This is a video tutorial from Eugene Opperman of musecreative.net on how to to prepare your models for upload on Sketchfab from Cinema 4D:
The final model:
You can also use our exporter to upload directly from C4D.
Here is a bit more info from Eugene for manual export:
Edit all that can be edited: Boolean symmetries, HyperNurbs — everything must be passed as polygonal. Then, for each polygonal object, click the Object menu (in the Object Manager) and choose Conform object. A window appears. Check illumination (if you rigged lighting) and unique texture, set the texture size (the bigger the texture is, the better the result, but the longer the conformation). Choose a path and JPEG format. Then click Conform.
Repeat for each object. You can also connect + delete your polygonal objects to do at once, but the result may not be as good. UV unfolding performed correctly is preferable, but not mandatory. Then select the object or objects created, copy (ctrl+C or ⌘C), create a new scene and paste.
For each material, copy the image channel in the channel luminescence color and luminescence disable the channel. In the File menu, save the project and export your scene in Collada 1.4 format. Zip the folder and send it to Sketchfab!