There’s no secret to it. All the info you need is in these forums. You mostly need to make sure that the […] in the following:
between your wp-config.php and bb-config.php.
Make sure you use different table prefixes for WP and bbP.
bb-config.php shouldn’t have anything extra defined in it past that, if you had left in WP2.8. Since you downgraded to 2.7 your bb-config.php needs to have
define('WP_AUTH_COOKIE_VERSION', 1); until you upgrade back to 2.8, at which point you need to remove that line again.
Your wp-config.php needs to have
define('COOKIEPATH', '/wordpress/'); where ‘wordpress’ is the path to wherever you installed WP, or
define('COOKIEPATH', '/'); if WP is in your site’s base folder.
Visit the WP administration settings and fill out the bbPress Integration Plugin settings (but mind what I said above, you probably shouldn’t add what it suggests adding to your wp-config.php file).
Visit the bbP administration settings and fill out the WordPress Integration settings there as well.
As I said in the posts I linked earlier, everything works fine for me with WP2.8-RC1 or any of the betas, and bbP1.0-RC1, except that if users log in from bbP, they can’t log out from WP (which seems a somewhat trivial issue since I doubt many users will need to do that).
Hope you can get it working.