The Storm Window
When deciding on a storm window, look for quality in construction. The first place to look is in the corners, they should be tight, secure, and not allow air to slip through. Also look at the weather stripping and the quality of the hardware.
If you have ever installed a storm door, you will have no problem with storm windows. Make sure you purchase the right size. The storm windows will attach directly to you stops, so you can use the dimensions of the opening at this point as a guide.
When you have ordered and received your storm windows, make certain that all old storm windows and obstructions have been removed. Depending on the manufacturer, you may have to drill pilot holes in the flange, some come pre-drilled. You may wish to squirt adhesive onto the stop before inserting the window. This will help secure the window, but should you ever need to remove the storm window, it might cause difficulty. Screw the window directly into the stops, securing it all the way around. Always start at the top when doing this to prevent the storm window leaning out and falling off the sill while you are at work.
Finally, caulk around the edge of the entire flange to stop drafts.
There are also interior storm windows that come in a variety of makes and models. While they do save on the gas bill, they often take away from the beauty of old windows.