Difficulty: Moderately Challenging
Step1
First determine if a storm door is a good idea for the entry way in question. In general, if there is no storm door installing one will only make things better. Even if you have a high quality interior door the storm door will add an extra layer of insulation and draft prevention. Also if your existing storm door is drafty, very old, or made of a poor insulator such as wood or especially aluminum you should consider replacing it.
Step2
Now begin taking measurements. You will want to measure the opening not the existing door. You may do this by opening your door and measuring floor to lintel and side to side. You may also simply measure the rough opening if there is not an existing storm door in place. If you suspect the opening is not strait you can take measurements at several points and check sides/top with a level. If the opening is far out of line it may be necessary to install boards or shims to correct before you place the new door. In general; however, this is not a huge problem as storm doors are built to fit a range of opening sizes and thus come with built in adjustments.
Step3
Now let’s go shopping for your door. You can find good selections at any home improvement warehouse or large hardware store. In general, the larger the store the better the prices and selection will be. I find that Home Depot has good quality doors at very reasonable prices.
Doors can cost anywhere from eighty to more than two hundred dollars. When inspecting a door open and close it a few times to get a feel for the hardware. Also check the seals both around the door and its glass. Most modern storm doors have glass that can be slid up or down to either allow or block air flow. Make sure that when closed the glass is well sealed with rubber, felt, or magnetic strips. There is no point investing in a brand new door that will be as drafty as an old barn stall. Also check the material and construction of the door. Some doors are very cheap, but remember that 1/8 aluminum panel will not do much for you. A good way to tell if a door will insolate well is to place your hand on it. If the material is cool to the touch the door is not a good insulator. (If the door conducts heat away from your hand it will conduct heat away from your home.) I find that the very low end doors are of horrible quality, but for a modest price increase you can get a huge initial jump in quality. A hundred dollars will buy you a solid functional door for a back or basement entry. The more expensive doors tend to be the ornate types one would want in a main entry.
When looking at door sizes remember that most doors fit a range of openings. As long as your opening is within the range you should be ok using only the material provided in the door kit itself.
Lastly, consider the manufacturer of the door. Is it a company you trust? Was the product made in America? This is not just a buy American pitch here. There is a real increase in quality and the prices stay reasonable. (Anderson has an American made door of great quality for about $95 at Home Depot as of the writing of this article.) Also, as the atlantic Monthly stated in its special China issue, the general rule of Chinese industry is that any thing worth doing is worth doing sloppily.
Step4
Ok now that you have your doors lets talk installation. Home improvement stores tend to charge from fifty to one hundred twenty dollars to install a storm door. If you are a novice and can get a good price on installation it may be worth it to you. That being said I feel that a hundred dollars is a horrid sum to pay for a simple storm door install. Remember this is much easier than a main door. I have seen complete newbies pull off fine install jobs on storm doors. All you will need are basic tools, some patience, and someone who does not mind reading directions. I know none likes to do this, but they really do help and are important in helping you understand how your particular door adjusts and hangs.
So to sum it up, if you can get a good price and do not want the bother it is ok to pay for an install; however, do not be afraid of trying this job yourself.
Enthusiast 
