When unloading a trailer that is not attached to a tractor with a forklift what should you do?

If it's a trailer with sliding tandems, ensure that the tandems are slid all the way to the rear. Then, you place a dolly jack under the front of the trailer and raise if up to the bottom of the trailer. This prevents the trailer from pivoting on the landing gear and tipping forward under the weight of the forklift.