My 9 year old came to me and asked if I could make him into a Lego Boy for Halloween. Of course I said sure I can try. Then he kept giving me more and more ideas .... this is what we came up with in the end.

Step 1: Parts and Pieces

1 sheet of 2" think foam insulation (lowes)
3 cans of spray paint (yellow, red, and blue)
1 can of primer
1 tub of drywall filler
various colors of paint of eyes (I used white, brown and black)
10 or so sheets of foam board for the body (I found this at dollar tree
a good imagination and TONS of time for trial and error

Step 2: Body

cut the foam board into the right shapes
i measured the Lego man and just blew up the dimensions to fit my son
I glued the edges
I taped the edges with linen tape used for pictures
then I painted the whole thing
when it was all painted I taped the edges with clear packing tape to make sure none of it would get messed up by him just being a kid

Step 3: Head

cut sheet of insulation into at least 5 or 6 12" round circles
glue together with spray glue
sand with rough sandpaper (I used a piece of the reverse cutout and glued sandpaper to it)
then sand again with 220 sandpaper
put drywall filler all over it with rubber gloves on and put a little water in it to make it more plyable
let it dry overnight
sand with 300+ sandpaper
fill again if necessary
prime and paint