Watch this video from This Old House to learn how to put in a storm door.
1 Use bevel square to establish angle of doorsill.
2 Mark doorsill angle onto the bottom end of the hinge frame.
3 Use hacksaw to cut hinge frame to length.
4 Screw hinge frame to edge of screen door.
5 Screw door hinge to side jamb of door opening.
6 Install remaining frame pieces around top and sides of door.
7 Attach automatic closers and handle to door.
8 Secure the safety chain to head jamb and to inside of door.
9 Seal around storm-door frame with silicone caulk.