Sherri Osborn at About.com explains that the materials needed for a Cousin It costume include a black hat, brown yarn, dark sunglasses and sturdy tape, such as duct tape. The length of the yarn must be enough to reach from the forehead to the ground, and Osborn suggests cutting the pieces to the correct length.
First, cut a 5-inch piece of tape, and place the yarn pieces tightly against each other on the tape. The costumer may place another piece of tape on top if necessary to attach to the hat. The next step involves placing the tape on the inner brim of the hat so the hair falls down. Osborn also mentions that placing the hat on a hat rack works well when attaching the hair. She recommends using lots of yarn for more hair coverage.
The Addams Family costume site recommends wearing black gloves as part of the costume, and it suggests that a derby hat makes a great hat. This site also recommends taping or gluing the fake hair that comes in front of the eyes to the edges of the sunglasses so that the materials do not obstruct the wearer's vision. Trimming some hair away for the eyes to create small eye holes also helps keep a line of vision open.