- Start with setting up a new backup and follow the wizard until you can select a place to store your backups
- Choose Amazon S3 based
- Fill out the highlighted fields as shown below with your credentials. Even if they look like a dropdown, you can type tekst.
- Fill out a unique bucketname and click on "Create bucket" or use one you created earlier.
- Complete the wizard

You can download Duplicati at:
http://www.duplicati.com But first: Create a daDup account