Things You'll Need
Instructions
Semi
Build the chassis of your truck out of flat plates. Make a base that fits under the cab, and extend it backward so you can attach more wheels. The base should be at least a 6-studs square, with a 2- or 4-stud extended section.
Locate six wheels of the same size and shape, which you can attach to the bottom of the vehicle. There are a number of different styles of wheel, so choose ones that you can connect easily to your model. Set these wheels aside for now.
Build the cab. Build up a tall base section, include a seat and steering wheel. Some vehicles have a flat nose, while others have an extended section which houses the engine. Decide which style of cab you're trying to build and assemble it.
Add doors so your driver can get in and out.
Attach a windshield and roof to the top of the vehicle.
Attach the wheels to the bottom. The back end of the chassis, which should be flat, usually has four wheels, and the remaining two are placed beneath the cab.
Trailer
Assemble a connecting mechanism. You can use a Technic pin and a plate with a hole in it, or find a number of alternative designs. Use one that will be fairly flat, and which you can use to remove the trailer from the cab.
Build the flat platform of the trailer. Use flat plates, and build something around 6-studs wide by 16-studs long. You can make a longer or shorter trailer if you prefer.
Decide what kind of trailer you're building. Some trailers are large boxy containers, while others are flatbeds which have railings on the sides. These trailers are used to transport different kinds of items.
Assemble the walls of the trailer. Use either bricks, or something that makes a good railing around the perimeter of the base. Build up the walls until they reach the appropriate height. Cover with a roof, if necessary.
Connect four wheels to the back end of the trailer.
Install the connection mechanism to the cab and the bottom of the trailer, at the front. The trailer should turn independently of the truck.