To start you will need to fashion the Lego Man's head. To do this you need a Sono Tube that will fit over your head comfortably. You can find the Sono Tube at any home improvement store. You will need to cut the tube so that the length is four inches past the top of your head. Then taking cardboard you need to cut out one round piece with flaps to make the cover of the Sono Tube. On top of the cover you need to affix a thin round plastic contain. Then paint the entire structure yellow. You will need two holes for your eyes, and a drawn on mouth.
To fashion the white shirt you will need cardboard, cut to make the Lego Man shape. Lego Man is boxy, but not square. The shoulders will be narrower than the waist. When fashioning the shirt you need a head hole and arm holes. Once the shirt has been created you will need to spray paint it white, and draw a collar and two pockets on the front. You will also need the red paint to create a tie. You will use your own arms in the costume. So you do need to wear a white long sleeve shirt to complete the cardboard shirt.
The Lego pants will require a bit of finesse with the heavy duty florist wire. Before you can do anything you need to start with the wire. The wire should offer a square leg with an area for the knee to bend. The width of the pants should be the same as the bottom of the shirt. The bottom of the pants will actually need to be fashioned for your shoes. In other words you want a thick platform at the bottom to make walking easier. To get more of an idea on how to fashion the pants see a picture of a Lego Man or get out your old toys. Once the wire has been created for the pants you need to cover them in the black material.
Lastly you need to create the hands. If you can find yellow mittens use them. Otherwise you can make your own mittens. They should be fairly thin, but warm enough for a cold night.