1) Download the .zip file
2) (Optional) modify or create a .htaccess file (see: http://www.bbportal.org/topic.php?id=8&replies=1)
3) Upload the files using the same map structure as in the .zip file
Note: If you don't want to overide any template files, you can also upload all the template files to your my-templates folder instead of the bb-templates one
4) Configure bbPortal (see: http://www.bbportal.org/topic.php?id=9&replies=1)
5) Surf to your website and refresh it
6) Done, that was easy wasn't it?
When upgrading from 1.0
Do the same as above, but also upload the original template-functions.php to your \bb-includes folder. Since bbPortal is a plugin, chances to the bb-includes/template-functions.php file are no longer needed.