I working the doors and window department, and I get this question all the time. Here are a your options, there are 3 different types of storm door installs depending on how handy you are.
Andersen storm doors with the 45 Minute Easy Install System come
pre-assembled for easy installation with only basic tools in as little
as 45 minutes. Minimal installation steps with no tricky measurements or
sawing. Your storm door is ready to secure into the door opening in
2 Hour Easy Install System
Storm doors with the 2 Hour Easy Install System come prepped for easy assembly and installation with only basic skills and tools in about 2 hours. No tricky measurements or sawing. Convenient spacer clips eliminate guesswork by insuring door is level and aligned in the opening before securing.
The 3rd type of storm door install involves cutting & drilling to install the door, these are doors that are the least expensive and take much longer to install, depending on your abilities.
With this website link, you can choose the door you want, get installation instructions, video on how the measure the right area.
Before you can install the storm door you need to know exactly where to measure, and measure the right location is critical.