Q: how do i back up my documents in I cloud?

You can't with Classic environment.  If you are talking a different environment, please post to the correct forum:

http://discussions.apple.com/docs/DOC-2463

I might add, I also wrote a backup FAQ*

http://www.macmaps.com/backup.html

Use Apple menu -> About This Mac to find your operating system.

* Links to my pages may give me compensation.

You can back up an iOS device to iCloud but it is not designed to back up a Mac. However, documents you save to iCloud Drive on your Mac are also saved to iCloud Drive at iCloud.com. That alone is not a good backup however because if you were to accidentally delete the document from your Mac it would also delete from iCloud. If you save the document to iCloud Drive on your Mac and if you are doing backups with Time Machine then it would be backed up locally on Time Machine and you would have a remote copy in case of fire or theft. More information on iCloud Drive:

http://www.apple.com/icloud/icloud-drive/

iCloud Drive FAQ - Apple Support