Storm doors are just about the easiest projects to get done well. So long as you have the measurements right and can read the instructions that come with the unit, it should be no problem for the typical DIY'er.

I don't know what molding you're referring to. Most storm units simply attach to the brickmold. You can download instructions in pdf format for a primer on the usual steps necessary when installing a storm unit. Go to Larson doors or anderson storm doors and others that come up on an internet search.

I'm looking to replace ours as well. At the store they told me to just measure the house door, measuring the storm door was confusing, measure the door or measure the casing. They have standards in stock. in my case I would need to trim out the bottom of the door, you can drop the bottom panal, trim door and put trim back on to cover the cut.

Usually the interior swing door and the storm door are mounted in the same size jamb, thus the same size doors.
Front door are usually 36 x 80 and side and back doors are 32 x 80. Once in a while you'll run across a 30" x 80.
But those are your most common sizes.
Yes it is true, you can measure the interior door and that is usually the size of the storm door.
Remember the door sill slopes downward towards the outside of the house and that is what the storm door bottom expander is for (U-Channel with slide in weatherstrip). It is used to accomadate the differernce in the height measurement.
The Z Bar is the frame that comes with the door. You get three pieces, the handle side, the hinge side and the top.

Its easier to mount the hinged Z bar right onto the storm door white laying on its edge. Be sure to let the top of that Z bar overlap the top of the door by 1/8" for door swing clearance. Measure the height where the door hinge goes. If it is 80 1/2", Measure down from the top of the hinged Z bar and cut it another 1/8" shorter to allow clearnace for the top Z bar to esaily go in - thats 80 3/8". Take note if your Z bar cut should be straight or slanted to fit on the sill. YOu cut the Z bar with a hacksaw.
After fitted, mount the door-Z bar combo onto the the brick mould. Just put in a couple of screws for now. Put the rest in later.
Now slide inthe top Z bar, closing the storm door over it.Create a equal space (the 1/8") across the top of the door - screw it down.
Now measure the the handle side sill to bottom of top Z bar and cut it with the hacksaw.
Rule of thumb, always cut the Z bar bottoms and leave the factory edge to the top.
Except for closer, hardware and safety chain, the door is in. Oh and don't forget. Always install the door without the glass or screen in the door. Ther're the last things you put in.

Good Luck

The PorchGuy

Here is a link that might be useful: My Album

Sounds totally do-able. Thanks, everyone for the advice!

Go to Seattle Door and they can show you how to replace. www.seattledoor.com

Porchguy, you have given what seems very good instuctions, thanks, in the process of putting new storm at sons,(forgot to bring hacksaw), will work on it next weekend, one question, instructions say to put top z-bar first, although with only one screw, you suggest side z-bar with door attached, you obviously wouldn't say if was harder to do, but this is easier to do? and thanks for the info.