Summarize the following:
Place the cap over the crown of the chimney, keeping it fairly centered over the top flue.  Since this type of cap essentially "cages in" the chimney, you do not need to worry about a perfect fit. Just make sure that the entire chimney is covered. Note that the "crown" of the chimney is the concrete area around the flues. Use heavy pliers to bend the flange around the bottom of the chimney cap. Keep bending the flange until it conforms to the outer perimeter of the chimney crown.  The "flange" is simply the collar surrounding the bottom of the cap. You need this flange to lie snug against the sides of the crown. Take a dark marker and trace around the lower, bent edge of the flange, making the mark directly on the chimney crown. After tracing the flange, temporarily remove the cap and set it aside. Generously apply weather-proof caulk or a similar adhesive to the crown of the chimney, positioning the adhesive just to the inside of the line you traced.  You can slather this on with a trowel, but the easiest way to apply the adhesive is to use a caulking gun. Make sure that the bead you apply is 3/8 inch (0.95 cm) thick. Move the caulking gun gently from side to side, creating a wavy line roughly 1 inch (2.5 cm) wide. Leave 1/4 inch (0.64 cm) gap at each corner as you apply the adhesive. Place the cap back over the chimney and firmly press the flange down into the adhesive. Let dry.  If desired, you could smooth any adhesive that oozes out using a putty knife, but doing so is not necessary. Oftentimes, weather-proof adhesive is enough to hold this type of cap in place. Some caps should also be secured further with masonry screws, though. Check the manufacturer's instructions to determine if screws are necessary. If they aren't you can stop at this step. Place a screw through each hole in your flange. Use a power drill fitted with an appropriate drill bit to twist the screws roughly halfway in place.  Work carefully. You will be drilling directly into the chimney crown, and if you are too sloppy or hasty, you could damage the masonry. Use a 3/16 inch (0.5 cm) masonry bit. The masonry screws need to be 1/4 inch (0.64 cm) in diameter and 1-1/2 to 1-3/4 inch (3.8 to 4.4 cm) long. You need at least one screw per side. Aside from that, the screws should be spaced about 12 inches (30.5 cm) apart. Go back around the flange and finish drilling the screws in place. The head of each screw should lie flat against the side of the flange. You should still work carefully to prevent any accidental damage to the masonry as you drill the screws in.

Summary:
Position the cap. Bend the flange. Trace the perimeter. Apply adhesive. Press the cap into the adhesive. Make pilot holes in the chimney. Tighten the screws.