How do I back up my macbook to iCloud

iCloud was not designed by Apple to be a Backup location for Apple Computers.

iCloud is intended as a conduit to have files on iCloud so All other Devices Owned by the User can access the same files.

Further, to make changes on one device will be reflected on All Other Device provided they are all using the Same Apple ID Account.

Time Machine Backup is one of the better options to Backup the computer.

A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. 

You can't.

You can enable Desktop & Documents in iCloud Drive, then everything in Desktop and Documents folders will be synchronized to iCloud. AppleID System Preferences, iCloud Drive Options.

Time Machine is the backup system for macOS. It is not cloud-based.

Time Machine Backup uses an External Drive that is oftentimes Directly connect to the computer and usually via a USB connection.

If using a USB Drive it is best to have the External Drive Capacity to be 2 to 2.5 Times larger that the Capacity of the Internal Drive.

Example: Internal Drive Capacity is 1 TB . The External Drive Drive Capacity 2 TB to 2.5 TB

As stated TM has nothing to do with iCloud. Get an external drive and use TM to do a complete backup.