All I did was follow the instructions on how to integrate a MyBB forum (
it is literally one paragraph on the official thread for the v1.3.4 release). There are just two important details when it comes to integrating MyBB -
one: Your site can't have any members besides you, else the process gets really messy -
two: The newest version of MyBB won't work. You'll either need to download v1.6 or tweak a few internal files so you stop getting the 'Failed to create forum account' error while trying to register someone.
(Note that above 'one' and 'two' are links that can help.)
All I did was create a custom template for MyBB that was identical to the template for my site. You'll have to look around on the official MyBB forums for instructions on how to make templates or insert your own custom header information. Modifying MyBB to look like your site, just learning how and where to do that, is a whole other animal. I'm afraid it's been too long for me to be able to help you.