When you insert the Leopard install disk and boot off of it you will be presented with a semi-Mac OS X desktop. In the menu bar, select Utilities and then "Restore System from Backup..." Select your backup drive, the date you want to backup from, and then click restore.
James Duncan Davidson has a full guide on his website describing how the process went. He mentions that while it restores all the files, the caches and databases are not restored. This means when you launch programs such as Mail the application will need to recreate the database, which may take some time.