Rule Number 1:
Any time you will be moving a vehicle onto or off from a trailer, the trailer must be securely hitched to the tow vehicle. There are no exceptions to this rule.

Rule Number 2:
Total weight of the trailer and the load combined should not exceed the "Gross Load" specifications for towing capacity of the tow vehicle. Also the amount of "Hitch Load" should not exceed the specifications for towing capacity of the tow vehicle. Any exception to this rule is very rare, and only in case of emergency, in which case you need to be very cautious about how to drive the complete rig, not to damage the tow vehicle and not to cause any road hazard. If the tow vehicle does not meet the requirements for Gross Load or Hitch Load towing capacity, I suggest you switch to a better tow vehicle. There may be some ways to upgrade towing capacity of the tow vehicle (helper springs or air shocks for instance), but that would be the subject of a different discussion.

Rule Number 3:
For proper handling on the road, a trailer needs to have the "Center of Gravity", with or without the load, slightly forward of the load axle, or a bit forward of the center between two axles of a tandem axle trailer. This will place an appreciable load on the hitch. Hitch Load should be about 10% of the Loaded Weight (gross load) for small trailers, or a minimum of 5% of the Loaded Weight for heavier trailers.

Rule Number 4:
Laws may vary by locality, but for sure heavy trailers must have Operational Brakes. For instance, in Illinois any road going trailer over 1000 pounds loaded weight must have brakes on one axle. Any trailer over 3000 pounds loaded weight must have brakes on all wheels. Trailer brakes in detail will be subject of another discussion.

Measuring Hitch Height and Spring Rate:
Any trailer must be built with appropriate Hitch Load for proper handling with the trailer empty (unladen). When loading the trailer, the load needs to be positioned such as to achieve the appropriate Hitch Load when fully laden. There is a convenient way to measure the amount of "Hitch Depression", and to position the car on the trailer to adjust load distribution and Hitch Load. Start with proper air pressure in the tow vehicle tires. Get a stick (paint mixing stick perhaps) long enough to reach from ground up to the tow hitch. With nothing connected to the tow vehicle, place the stick by the hitch, and make a mark on the stick at height of a fixed reference feature on the hitch. Get a friend of known weight (200 pounds for instance) to stand on the hitch, and make another mark on the stick to denote the new depressed height of the hitch. You can measure the distance between the two marks, then divide weight by distance to calculate Spring Rate of the hitch. For instance if a 200 pound friend causes it to settle 1/2 inch, then the spring rate is 400 pounds per inch. When you know the hitch load desired for the loaded trailer you can do the math and make a third mark on the stick in the proper place to represent the intended final loaded hitch height. Keep this stick handy when loading the trailer for the first time, and you may want to keep the stick in the tow vehicle at all times in case the trailer load might change. More about this later.

Loading the Car Onto the Trailer:
Hitch the trailer securely to the tow vehicle, and load the car onto the trailer. Loading a car onto a tilt bed trailer is covered in general in prior articles featuring the tilt bed trailers. In essence, tilt the load bed to touch the ground in back. Drive the car (or winch it) onto the trailer until the bed tilts back down to level where it can be latched in place. A fixed position flat bed trailer is a bit more involved for loading. The first requirement is a pair of ramps that can be affixed to the tail end of the trailer, after which the car can be driven or winched up the ramps onto the flat load bed.

A common issue here is that the ramps may be relatively short compared to the trailer bed height. There may be a rather steep ramp angle that may cause some problems. Tail end of the car might hit the ground about the time the front wheels are approaching the load bed. Also the center of the car chassis may get hung up (high centered) on the top of ramps or tail end of the load bed.

One solution to this is to raise the front end of the trailer (tilt slightly) to drop the tail end closer to the ground. This will reduce the ramp angle and also reduce the apex height between ramp and load bed. You may accomplish this by putting the trailer tongue jack down and cranking it up to raise the hitch, also raising the tail end of the tow vehicle in the process. There is a practical limit to this maneuver, limited by lifting range of the tongue jack, and the idea that you don't want to lift the tow vehicle wheels off the ground. With the trailer tilted slightly in this manner the car may be driven or winched onto the trailer without the car tail touching the ground and without being high centered on the apex of the ramp. If this is not enough of a change, then you may need longer ramps.

Adjusting Load Position and Hitch Load:
Once the car is on the trailer you should park it with center of gravity approximately over the load axle, or midway between axles of a tandem axle trailer. Then place your reference stick near the tow hitch, and move the car fore or aft on the trailer until the hitch height matches the desired reference height mark on the stick, thereby setting the desired hitch load. If you will be carrying loading ramps, car parts, tools, fuel cans, or any other weighty items on the trailer, these items should be in place when you are positioning the car for the desired hitch load.

Once the car is properly positioned on the trailer, make up some blocks to be placed against the front of the car front tires, and firmly attached to the trailer bed (permanently) to serve as stops so the car cannot roll forward. I suggest three layers of 1-1/2 inch thick boards for total height of 4-1/2 inches. You can then connect the winch hook onto the car chassis, pull the car forward tightly against the stop blocks, and latch the winch in that position with the strap or cable in tension.

Tying The Car Down on the Trailer:
At the rear of the trailer you need a long web strap for the rear tie down. You can pass the strap under the rear axle, then loop it up and over the nose of the differential (assuming rear wheel drive car), then down the other side and back under the axle again. Connect the ends of the strap to a pair of points at the rear of the trailer bed, somewhat outboard from the center (like a big triangle), and tighten the strap securely. This avoids smashing brake pipes that run along the rear axle housing. The idea here is to tie down the chassis and axles while allowing the car to ride on the suspension, same as if it was driven on the road. The car's suspension action will make the car ride smoothly, and at the same time will help to dampen trailer bed motion to make the trailer ride smoother on the road. Action of the car suspension will also eliminate shock loads on the tie-down straps that would be caused by harsh bumps (if you tried to tie the car body down tight).

You will also want some provision to prevent the car from moving sideways on the trailer. Cage style wheel straps work well for this purpose. Place the wheel strap over top of a tire like a cargo net, fasten the end hooks to the trailer bed, and pull the strap tight. You can do this for two diagonally opposed wheels, for instance right front and left rear. This will tie down both front and rear axles, in case something really dramatic might happen, such as an emergency maneuver or a side impact (or harsh tire to curb contact).

Finally, keeping in mind that the only front end tie you have so far may be the winch cable, you should install a Safety Chain from front of trailer to the car chassis. This could be a taught web strap or a snug chain with a connecting bolt through the links. If the winch latch should fail you don't want the car rolling fore and aft on the trailer bed.

By this time you have "Belts And Suspenders", or redundant links in all directions for security and safety. Front block and rear strap prevent forward movement. Winch cable and safety chain prevent rearward movement. Two wheel straps prevent sideways movement and prevent the car from jumping upward in case of a severe road bump. The wheel straps also backup all other tie-down attachments, and vice versa. And the car is still free to ride smoothly on its suspension. Then load up and fasten down the loading ramps, plug in the trailer harness, check all of the running lights, and you are ready to roll test the trailer brakes before hitting the road in earnest.

I used to have photos of our old stock car on a flat bed trailer. I will scan and post them if I can find them. Meanwhile, if anyone does this, and will take pictures, I would appreciate it if you could send along the matching photos.