A storm door protects your main entry door from weather, adds an insulating air gap, and lets you open up for ventilation with the screen. Most are sold as kits designed for DIY installation.
Measure carefully
Measure the door opening width and height precisely, and buy a storm door that matches — they come in standard sizes with adjustable frames. Note which way your door is hinged so you get the right handing or a reversible model.
Mount the frame
Storm doors mount to the exterior door frame (the brick mould or trim). Attach the hinge-side mounting strip first, level and plumb, hang the door, then fit the latch-side and top strips, trimming as the instructions direct for a snug fit.
Adjust the closer and sweep
Install the pneumatic closer and adjust it so the door shuts firmly but not violently, and add the safety chain that stops wind from flinging it open. The bottom sweep should seal against the threshold. Test the latch and weatherstripping.