You must cancel in writing. The seller must give you a written notice telling you about your right to cancel the agreement, plus two copies of a cancellation form. If you want to cancel, you can mail or hand-deliver one copy of the cancellation form to the address given for cancellation. Keep the second copy for your records. If you were not given a form, you can write your own letter.
If you cancel by mail, it is best to use certified mail so that you can prove when and to whom the letter was mailed.
If the seller did not give you written notice of your right to cancel and copies of the cancellation form when you signed the contract, then you can cancel it until three days after the seller finally gives you these documents.