How do I back up my iCloud

You don't really backup your iCloud. iCloud itself is a backup service. Essentially you are asking "how do I backup a backup." What is it you really want to backup?

How to back up your iPhone, iPad, and iPod touch with iCloud - Apple Support

Do you mean how do you backup to iCloud? See if this helps: How to back up your iPhone, iPad, and iPod touch - Apple Support