Use the flippers, and however much nudging you like, to guide your balls. As long as your base is behind a building you are safe and any lost balls will be returned. However, if an invader reaches the bottom and turns red, they destroy that building and a lost ball will result in a lost life. Keep the bottom row clear as much as possible. If you hit a UFO the bottom row is cleared.
After the first wave, all but the bottom row will retreat to the top of the screen. The second wave increases the number of invaders that can appear each time. At the end of the second wave, you must clear all the invaders, but beware of UFOs dropping off reinforcements.
Once the Boss appears multiball is launched, one ball for each life you have. Lose a ball and that life is lost. Break your way through the shields to destroy it.
You get an extra life at the end of each wave, when you clear the last invader, and for beating the Boss.
If you prefer a more video game-style view (as in the 'flat' invader screenie blow), check Table Info before running.