I'll show you how to use your Dropbox account as a secured storage for your websites and databases backups.
Nothing simpler than that actually.
Connect to your SimpleBackups account, and head to the "Create new Storage" section.
Select "Dropbox" in the available storage options
In the "Finalize and create" section, simply define the name of your storage (ex: My Dropbox)
Hit "Connect Dropbox", you'll then be redirected to a Dropbox page asking you to grant access to your storage to SimpleBackups - Login and hit continue
That's it your Dropbox account is not linked to SimpleBackups.
Note that your backups will be stored in "/Applications/SimpleBackups.io"