For Player 1 (the thing that's red and yellow) use W to move forward, A to turn left, D to turn right, and S to shoot a lazer (shooting a lazer takes one energy away). For Player 2 (the thing that's blue and green) use UP ARROW to move forward, LEFT ARROW to turn left (duh), RIGHT ARROW to turn right (duh (again)), and DOWN ARROW to shoot a lazer, (remember, shooting a lazer takes one energy away). A green energy ball will appear once in a while, collect it to get energy. The white lines coming out of your character is just an aiming feature to show you where your shot is going to hit (in other words, it helps you aim). Hit your opponent with lazers to defeat them!
Music by Undertale