Start from the emtpy game template
Here is how your games looks by default when it has just been created :
Setup the board
Gather useful images for the game and edit them as needed.
Edit .tpl to add some div for the board in the HTML.
Edit .css to show the image of the board as background.
Setup the backbone of your game
Edit .sql to create a table for intersections.
Edit .game.php->setupNewGame to insert the empty intersections (19x19) with coordinates into the database.
Edit .game.php->getAllDatas to retrieve the state of the intersections from the database.
Edit .tpl to create a template for intersections (jstpl_intersection).
Edit .js to setup the intersections layer that will be used to get click events and to display the stones. Use some temporary css colors on borders or background+opacity to make sure they are positioned right. You can declare some constants in material.inc.php and pass them to your .js through .game.php->getAllDatas for easy repositioning.
Manage states and events
Define your game states in states.inc.php
Add onclick events on intersections in .js, calling an action with appropriate parameters
Add action in .action.php, retrieving parameters and calling the appropriate game action
Add game action in .game.php to update the database, then notify the client using a method ‘stonePlayed’
The basic game turn is implemented: you can drop some stones!
Remove temporary css visualisation helpers : looks good!
Implement rules and end of game condition(s)
Implement specific rules for the game (if any)
Nothing special for Gomoku
Implement rule for computing game progression in .game.php->getGameProgression()
Implement end of game detection and update the score according to who is the winner in .game.php->stCheckEndOfGame()
Notify the score and implement the corresponding interface update in .js
Test everything thoroughly... you are done!