The basics.
To use the flipper you have to press either the left or the right shift key (this is the default) to move one of the flippers.
Here are a few examples for using the flippers:

• Launching the ball.
  
• Holding the ball back.
  
• Moving them whilst bored.

When to use the flippers
Press the shift key to shoot the ball away from you.

The shift key is a modifier key on a keyboard, used to type capital letters and other alternate "upper" characters. There are typically two shift keys, on the left and right sides of the row below the home row. The shift key's name originated from the typewriter, where one had to press and hold the button to shift up the case stamp to change to capital letters; the shift key was first used in the Remington No. 2 Type-Writer of 1878; the No. 1 model was capital-only.[1]

On the US layout and similar keyboard layouts, characters that typically require the use of the shift key include the parentheses, the question mark, the exclamation point, and the colon.

When the caps lock key is engaged, the shift key can be used to type lowercase letters on many operating systems, but not OS X.
Source[en.wikipedia.org]

Practicing a lot.

Using the flippers.

Friends who support you. (This is one of the most important steps)
For example:

Originally posted by Scout Meister:

You can never do it, this game is for losers.

Originally posted by Nova Meister:

LOL, you are a noob. I just beat your highscore, loser.

Profit.

Thank you for visiting, I hope you have learned something out of this.