Installing a pre-hung door — that is, a door that is already hanging in its own frame — simplifies the installation process, but that doesn't mean it's easy. You'll still have to adjust doors and jambs to compensate for imperfections in a wall frame.
Before buying a pre-hung door, make certain you know which side the hinges should be on and which direction the door swings. You'll select a right or left hand door based on this. You also need a precise measurement of your rough opening. You should leave a clearance of 5cm over the height and 5cm more than the width of the frame. This will allow for final door adjustments to get it square by adding shims. Any gaps around the door will be covered by the door casing.
Check out the video above, from Fine Homebuilding, for complete instructions for installing a prehung door. Note, the video example uses a plumb and level door opening. You won't always be so lucky.
Install a Prehung Exterior Door [Fine Homebuilding]