Tips and advice for installing a chimney cap?
For a typical masonry chimney installing a chimney cap is something you can tackle yourself. However, do not try to fabricate a cap yourself. A chimney cap is not something that you want to make at home. The cap you purchase should come with the proper fasteners which will usually be screws that are pointed on the ends to penetrate the flue liner that is exposed at the very top of your chimney. Some caps have a flue built in that requires a cable to run down the length of the chimney. Unless there is a problem with your fireplace I would not use one like that. The materials used to construct the cap are important. Don't buy one that could rust and consider a finish that will never need repainted.

While you are up there take the time to inspect your chimney inside and out. Look for any areas that are missing mortar. Check the inside for shiny black covering or cracks. Make sure there are no missing bricks and the top of the chimney properly sheds water. Any of these issues and I recommend brining in a professional.