Default 'Manual' Method of taking shots:
Aim with the mouse, hold down the left mouse button and move downwards to set the power. Use the scale that appears on the left to judge the power more accurately.

'Timer' method (Useful for some challenges!):
Aim with the mouse. Hold down the left mouse button to build-up more power before letting go to take a shot.

Shot styles can be changed at any point using the in game menu in the top left corner of the screen.