Collect the parts you need for this project. Part numbers are from http://www.bricklink.com/2x3961 Radar Dish 8x81x3941 Brick, Round 2x21x4032 Plate, Round 2x2 with Axle Hole1x32073 Technic, Axle 5(Optional) ...
Attach the round brick to the inside of one radar dish. Attach the round plate to the inside of the other radar dish. The brick and the plate each have a hole for the axle which allows them to be joined together The two round pieces will be hidden inside the body of the UFO and offer a little bit of extra mass to help the top spin.
Place the two radar dishes together such that they form a standard UFO body. Insert an axle into both dishes to form a top. Push the axle through so that it sticks out a little on just one side (which is the bottom).
Place the dome on the top of the UFO. You can add your favorite minifig head inside the dome. You can also substitute decorated radar dishes or other colored parts as you see fit. And it's a top, so you can give it a spin and watch it turn.