Final answer:
The first step in installing an exterior door is to prepare the door opening for installation, ensuring accurate measurements are taken to facilitate a proper fit for the new door.
Step-by-step explanation:
The first step when installing an exterior door is preparing the door opening for installation. To ensure a proper fit, the door opening must be measured accurately. Choose a unit of measurement suitable for the size of the door, such as inches or centimeters. It is important to get this step right before proceeding to remove the old door and frame, measuring, or cutting the new door to fit, and finally installing the door and frame. Attention to detail during the preparation phase will set the stage for a successful installation process.