Before uncoupling a trailer, what must you do first?

Before opening the fifth wheel release: position the trailer where it will be parked (on stable, level ground if possible), lower the landing gear until firmly on the ground (then crank a few more turns to take weight off the tractor), and make sure the trailer cannot roll. If the trailer has spring brakes that engage when air is released, those will hold it. If not, chock the wheels. Only then disconnect the air lines and electrical cord and release the fifth wheel locking jaws. Pulling out from under a trailer whose landing gear is not lowered will cause the trailer nose to drop several feet, possibly damaging the trailer, cargo, the fifth wheel, and anyone nearby. The order matters.