Multiply the diameter of the dryer vent pipe by pi (3.14) to get the circumference of the dryer vent pipe. Your dryer vent piping is likely 4 inches in diameter, so your circumference will be around 12 1/2 inches.
Add 6 inches to that measurement to allow for the thickness of the insulation and to allow for appropriate overlap.
Roll the insulation out on the floor and cut at the above calculated measurement.
Wrap the insulation around the dryer vent pipe and pull it tight. It should overlap a few inches.
Tape the overlap seam down with 3-inch foil aluminum tape. Fasten the tape to the entire length of the seam.
Continue to cut insulation and secure it to the dryer vent pipe until the entire dryer vent pipe is insulated. Tape all seams including where the separate pieces of insulation butt up.