THE TOWING SETUP
Before hooking trailer to tow vehicle, walk around each to check that they're fit for the road. Make sure the tires are inflated correctly (look in the owner's manual for tow-vehicle tire pressures, on the tire sidewalls for the trailer), and that hoses, belts, fluid levels, trailer spring hangers, and springs are in good shape. All cargo and gear must be stored securely.
Make sure hitch, drawbar, and trailer ball are the proper ones for the trailer you're about to tow--and that all are tight. The size of the required ball is stamped into the body of the trailer coupler and the ball itself has its size stamped into the top.
This will take less than 10 minutes and can eliminate the vast majority of trailer problems that occur on the highway.
Drop the trailer onto the hitch ball, then lock the trailer coupler lever and place a locking pin or other bolt through the lever to keep it from accidentally popping open while you're driving. Attach the safety chains by crossing them under the coupler and hooking them onto the hitch loops in the proper orientation. Then attach the breakaway brake cable to the hitch.
Step back and observe the tow vehicle and trailer from the side: The trailer should sit parallel with the ground (or ever so slightly tongue low) and in line with the chassis of the tow vehicle.
If the trailer tongue is too high or too low, the load on/in the trailer may be too far forward or rearward, which will adversely affect how the trailer tows. Move the weight on the trailer until the level balance is achieved, adjust the spring bars on the W-D hitch to better balance the load, or change the hitch shank to one that brings the tow vehicle/trailer into proper alignment.
Insert the plug on the trailer harness into the receptacle on the tow vehicle. Test the turn signals and brake lights to make sure they're working on the trailer. When trailer and tow vehicle are properly set up, adjust the mirrors so you see down the entire length of the trailer.