The overall goal of getting Fireboy and Watergirl to exit together through their marked doors remains the same, as does the importance of grabbing the red and blue gems wherever you can, and of course, you'll want to do it as fast as possible. The number of gems you grab and how fast you do it combines to give you your score. Even if you're not ambidextrous, most of the levels can be completed playing one character at a time, although naturally that way is slower. You can also make this game into an unofficial local multiplayer game if you can get a friend to control one half of the keyboard. Laptops with external keyboards plugged into them work especially well for playing as a team; one person takes the laptop keyboard, the other the external one. Not that I have any reason to know that. Ahem.
Arrow keys to control fireboy.
WASD to control watergirl.