It's not really a tutorial but a good starting place would be phpBB's official modding manual - phpBB • phpBB Modifications Documentation

It's been (semi-) updated for phpBB3 and is laid out in a book-format so is fairly easy to follow.

Also, the phpBB3 mod forum should have some useful info - phpBB • View forum - [3.0.x] MOD Writers Discussion

Good luck :)

Alan