...all (666 or -rw-rw-rw- within your FTP Client)
Change the permissions on the following directories to be writable by all (777 or -rwxrwxrwx within your FTP Client):
store/, cache/, files/ and images/avatars/upload/.
Using your web browser visit the location you placed phpBB3 with the addition of install/index.php or pointing directly to install/, e.g. www.mydomain.com/phpBB3/install/, www.mydomain.com/forum/install/ etc.
Click the INSTALL tab, follow the steps and fill out all the requested information.
Change the permissions on config.php to be writable only by yourself (644 or -rw-r--r-- within your FTP Client)
phpBB3 should now be available, please MAKE SURE you read at least Section 6 below for important, security related post-installation instructions.
If you experienced problems or do not know how to proceed with any of the steps above please read the rest of this document.