I took foam the kind you have left over from siding and made the circles for the head, 2 larger, 2 slightly smaller and then used 2 pieces of poster board from the dollar store. I used yellow paint which worked out well for me because I had it on hand. The eyes I used duct tape and the mouth I trimmed with electrical tape.
The body and legs were from boxes I got from my children's school. The legs specifically were 2 copier paper boxes. I bought 2 cans of black spray paint for under $2 a piece. I sprayed the body and legs down 3-4 times each drying all the way in between each spray. With white paint I had on hand I painted the white circle on the main body.
The lego hands I used 4 layers of the foam like I used for the head and bought caulking glue and glue them together just like the head pieces. I used a drill saw to shape the dried foam into a pac man like shape and then again so it's kind of pac man but not, hard to explain, but you get the idea. Then I painted them yellow as well. I wore dark blue jeans and a long sleeve black shirt with it. The mouth is what I could see out of when I wore the full costume.
This was easy to make, lots of fun and fun to wear. The kids got a big kick out of it too on Halloween night!!