My son wanted to be Emmet from LEGO MOVIE for Halloween so I looked all over for the costume @ every retailer imaginable, after I had run out of options I decided that I would try to make his costume at home. The head and hair are just a sheet of dense foam. I cut out many of the needed diameter and glued them all together. after that had died I used a coarse wood file to start shaping the foam to my desired shape. I then coated it in resin and painted it. The whole body is nothing more than precisely cut junk cardboard scraps taped together and covered in resin.
First thing you need to do is get some measurements. You want the costume to fit the person you are building it for. You can't expect to build it and just hope they can fit in to it. Get measurements of head, shoulders, chest, mid section, waist, legs, feet, etc.. I took every measurement I made and added one inch to it. That way the costume would fit "snug". I found a flat piece off cardboard and started drawing out my measurements. I decided how wide the waist needed to be to look the most "lego like", then based everything from the waist down off of those measurements. Having a lego figure with you will really help with this step. I drew it out with tabs so that it didn't have gaps on the corners after I cut it out and folded the cardboard.