Use the arrow buttons to move Fireboy and Watergirl. When you are running, the other direction becomes a jump button. The arrow in the middle tells you which character you are currently controlling. Tap it or either character to switch between Fireboy and Watergirl. Walk through portals to teleport. You can also push objects through them. White portals are always connected to other white portals, and black portals are connected to black portals.
Use the keyboard to move Fireboy and Watergirl through the maze and collect all the diamonds on their way to the exits. The A,W,D keys move Watergirl and the arrow keys move Fireboy.
Since fire and water don't mix, be sure to not let Fireboy go in the water lakes and don't let Watergirl go in the fire lakes... And keep them both out of the black lakes.