What is the best method to apply a screen protector to a touchscreen?
First I make sure the screen is completely clean using a micro-fiber cloth, then I use a credit card with the cloth wrapped around to press the screen protector tightly as soon as it touches the screen.

The trick is to press, making a forward/backward movement, the protector against the screen as soon as it touches it, removing any bubbles in the process. So don't try to put the protector all at once.