How to install MODs in phpBB3
MODs are modifications with which you can add new functions and features to your phpBB3 board. There are mods for many different things. You can browse the Modification Database on the official phpBB site to find new MODs, or you can use the search function.
The administration control panel doesn't have a built-in automatic installer for MODs, so the installation process involves mainly tasks that you have to perform manually: e.g. editing the code of files, uploading new files, executing SQL queries, etc. There's, however, a MOD that you can install and use it to install other MODs automatically. It's called AutoMOD. This is an article on how to install MODs manually. To learn how to do it using AutoMOD read the tutorial on installing and using AutoMOD.
Before you start the installation of any MOD you should create a backup of the phpBB3 files and of the database used by the application.
After you choose a MOD that you want to install, download the archive to your local computer and unpack it. The unpacked archive should contain an install XML file and optionally one or two folders with additional files and instructions. Open the XML file with you internet browser. It contains detailed instructions on what you have to do to install the MOD. If there are new files that you have to upload to the phpBB3 directory on your hosting account, it will specify the names of the files where you can find them in the unpacked MOD folders on your local computer, and where in the phpBB3 directory on your account you have to upload them.
The install XML file should also contain instructions on which files that are already on your account you have to edit. It will specify the particular file, the path to it, the code in the file that you have to find, and it will also provide some new code and what you have to do with it: e.g. add it before or after the specified old code that you have to find, replace the old code with the new one, etc. To edit the files you can download them using an FTP client, then edit them with a text editor (e.g. WordPad, Notepad), save the edited files, and upload them back to the same location with the FTP client.
The install XML file may also instruct you to perform some other tasks, such as SQL queries and DIY tasks. In case you have to perform SQL queries you'll be provided with the particular query that you have to execute. To execute the SQL query go to the Databases section of the Pixie control panel, open phpMyAdmin from the link on the right, open the database used by your phpBB3 application, click on the SQL tab at the top of the database tables, copy/paste the query in the text field and click on the Go button. If your database tables have a different prefix than the default phpbb_, you'll have to change it in the SQL command provided by the install XML file.
As for the DIY tasks, they are usually something simple that you have to do, such as purging the phpBB3 cache, changing some settings in the administration control panel, opening a script with your browser, etc.
For more details you can also check out the phpBB3 MODs Tutorial.