The manual tab on the Gemp page tells how to make decks and play the game.
You play simply by clicking the applicable cards when it is your turn for an action. For example, during your Fellowship phase, every card that you could play or use during that phase is highlighted. And when you are done performing actions, click Pass and you will move on to the next phase. All of the rules are enforced, all strength, twilight and archery totals calculated automatically. It's a very simple interface.