Get a bit of plywood and cut it to one foot by 3 foot.
Get a plastic single gang box and a plastic round light box and screw them to the plywood.
Get 6 feet of no. 14-3 SO or SJO cord and a Cord cap.
Get 6 feet of 14-2 w ground romex.
Put the cord cap on one end of the extension cord. Remember that the hot wire (black) goes on the darker color screw, and the ground wire goes on the green screw.
Get a single pole switch, a dimmer, and a keyless light fixture and bulb.
run the so cord and romex into the single gang box.
You will also need some electrical tape, a few wirenuts, and some staples.
splice the white wires through the box. Splice the grounds through and leave a pig tail for the ground screw on the switch.
the black wire from the cord and the black wire from the romex one to each of the screws on the switch. Wrapp them clockwise around the screws.
Run the other end of the romex into the light box. The white wire ties to the silver color screw and the black to the darker color screw. (white to the screw shell of the fixture, black to the center pin of the fixture.) connect the ground wire if the fixture has a green screw.
Staple the cables neatly down to the plywood. do not drive the staples too far.
Ask an adult to check your work, and have them be there when you plug in your project.
After you check your work, and before you plug it in, screw the light fixture and switch down to the boxes.
Plug in your project and check the switch works properly.
Next unplug your project and take out the switch. Connect the same three wires from the switch to the dimmer. Screw the dimmer down to the box, and ask the adult to help you plug the project back in.
Check your dimmer for proper operation.
As suggested mention that one must turn off power At The Breaker Panel before doing any electrical work. Use unplugging the cord as your way of insuring that you are not working on a live circuit. Re state that one should NEVER work on a live circuit.