When backing a tractor with a single trailer, you should:

When backing a tractor-trailer, the trailer goes in the opposite direction of your steering. Turn the wheel left and the rear of the trailer drifts right; turn right and the trailer drifts left. This is the opposite of backing a passenger car because the trailer pivots on the fifth wheel. New drivers find this counterintuitive. The technique: back slowly so corrections happen at low speed, use small steering inputs (large inputs cause oscillation that is hard to recover from), and use a helper when possible. When the trailer starts to drift in a direction you do not want, turn the steering wheel in the same direction the drift is going, which actually counters it by pushing the tractor in that direction.