Here's what I did to install and get a phpBB3 (RC4) site up and running:
0. Installed it just like another LAMP application.
1. Copied /styles/subsilver2 to /styles/bxl. So bxl is now a new theme. I selected it as the default theme for the site by clicking "install" against "bxl" from "Styles [tab] > styles" and then selecting it as the default theme.
2. Edited all *.cfg files (4 in total) under /styles/bxl/
3. Make necessary changes to /styles/bxl/theme/style.css.
4. Uploaded mylogo.jpg to /styles/bxl/imagesets/
5. Logged in as admin and went to "Styles [tab] > imagesets" and clicked "edit" against "brixlets".
6. Clicked select for "Main logo". Now the main logo is shown. I selected mylogo.jpg from the selection list below and entered the value of the width and height of the logo in their corresponding box. Next I clicked the "Submit" button. This completes the logo change.
7. To change the header content, I edited /styles/bxl/template/overall_header.html.
8. Logged into the site as admin and from "General [tab] > board settings", disabled "Enable birthday listing".
9. From "General [tab] > User registration settings", selected "By Admin" as the account activation method. Now all account creation must be approved by the admin of the site.
10. Entered the name of the site, slogan, etc in "General > Board settings".
11. Added a forum from the Forum tab.
12. From "Forum [tab] > Forum permissions", added users to forums of choice. Also, gave the guests read-only access to the site.
Overall impression is that phpBB3 has tons of features, but the user interface is below par.
Here's a useful reference on phpBB3 theming - http://olympuswiki.naderman.de/Templating_Tutorial