Last time I remember MODs being hard to install for phpBB was during 2.x, since 3.x it has been relatively easier.
1. Open the file that needs to be modified
2. Find the code that needs to be modified
3. Repeat 2 for every line that needs to be modified
4. Repeat 1, 2, and 3 for every file that needs to be modified
5. Hope that your forum still works
6. Find that your forum does in fact still work
Supposedly they are working on automatic installations in 3.0 (they did start in 2.x, but it was for like 3 mods and it did not work)
On an unrelated note, I remember trying out a different forum system some years ago and they had a package manager-like system for MODs. It would pull a list of mods from the site when you opened up the MOD controls in the Admin options. It would give you a list of all your installed MODs if you wanted to uninstall one, give you a list of new versions, and let you browse the available MODs. It was a one click install, upgrade, and uninstall system. Worked great. The only problem was that they very few MODs and none that were any good.