The letter could be placed in a freezer for approx 1 hr. This allows the envelope to be easily opened without damage. Prior to placing into the freezer, all Questioned Document departmental work should be completed.
to do anything you need to know what you are hoping to achieve, what kind of evidence are you looking for. For example, if you were not concerned about DNA/fingerprints, but wanted to compare handwriting you'd just open it. Start off with how serious the "crime" is, how the letter arrived, what your fictional team are hoping to achieve.