Lego charactors have been done a lot, but I wanted to do a scientist version to match my profession. The hardest part was definitely trying to do something which is supposed to have a gloss smooth finish everywhere. Hopefully some of these steps help you avoid headaches and create an amazing version of your own.
The scientist in me wanted to take this seriously. I found blueprint drawings of an actual lego mini-figure and you can see from the blueprint overlay that I hit the scaling pretty close.
THE FACE / NECK
[NOTE: Some pictures show a styrofoam face cylinder. Do not do this. It adds a lot of weight, thickness, mess, and it is very hard to glue or paint since solvents dissolve styrofoam. I actually melted my entire first head when I tried to use contact cement to attach a finish surface coating.]
This technique will apply to whatever head design you personally choose to do. Start with the cylinder/face portion of the head. Buy 12" cement form tubes that are made of a coated cardboard. Cut two segments that are ~22" long. Cut each piece lengthwise and spread them into a semi circle. You will overlap these two pieces, add gorilla glue, use heavy duty staples and clamp the joint. Touch your two pointer fingers and two thumbs together in a circle to see what I mean.
The mini-fig has a rounded "chin" which is made with a doughnut of dense core foam that you can cut with a jigsaw. Sand it or use a dremel to get the radius correct. The inside diameter of the doughnut should be 12" (big enough for your head to get through. You can put a 4" length of 12" cement form tubing to give rigidity to hold up weight of the head. Again, use gorilla glue, clamps, and let it cure for no less than a day.
Once it is cured (donít rush it), you can cut out a mouth hole to look out of. I then took foam similar to that of yoga mats and glued it to the cylinder using contact cement. Search for "foam shop" in your area to find the right stuff.
Apply one or more layers of screen door fabric from the interior of the head to cover the mouth, but allow for visibility.
Paint: Krylon makes a water based spray paint that wonít dissolve the foam rubber. Get the white color and spray several light layers on the head. Once it is sufficiently sealed you can go over it with a glossy regular spray paint of the perfect shade of lego yellow. I think Krylon has a "sunflower" color that came closest.
I did the eyes with sharpie, glasses with poster board, and electrical tape works very well for black details as it wonít pull the paint off if you mess up.
YOU HAVE TO DO THIS!!!! Buy or salvage computer fans. They are $2 on Amazon so get a couple. You will attach it to a 9 volt using a wire connector available at any radioshack. THIS WILL SAVE YOU AND MAKE THIS COSTUME BEARABLE!!!
I personally cut a hole in the top of the back of the head, hollowed a passage under the back of the hair, and it formed a good convection pulling air in the mouth.
Start with the dense home insulation foam that you can find at Home Depot or Lowes. Do not get styrofoam or bead board. It is way too messy, not structural, and you'll never get the smooth finish required to get a nice final finish. Depending on your design you can add wire "rebar" to reenforce thin areas.
Shape in 4 stages starting with hot wire cutting. You can get a hot wire tool online but they are extremely expensive. I salvaged mine by pulling the wire out of a blow dryer I found at a thrift shop. I wrapped the ends of the wire around bolts. I then grabbed each bolt with the terminal of a car battery trickle charger to head up the wire to a red hot temperature. It cut like a knife through butter but there are a lot of fumes so be careful.
You can then move to an electric carving knife followed by a dremel tool with a rounded pummice bit. Finally you can sand and smooth with dry wall spackle. It all depends on what youíre doing (chef hat, cowboy hat, space helmet).
The body wasnít bad. Obviously get cardboard. Large moving boxes from Home Depot worked great. First they are new so they are dry and smooth. It isnít worth it dumpster diving to save $0.98. Cut the box so it unfolds out into a flat sheet. Draw the chest trapazoid based on your shoulder width and torso length. Add 1-2 inches on each outer border which youíll fold over to glue to the other panels. The sides are obviously just rectangles of cardboard. Feel your inner fine italian tailor and have fun with this.
Next you need the glossy perfect surface. Use spray adhesive to attach poster board. Then finish it off with a coat of clear coat spray paint. At each step be careful to not get the poster board too wet so it doesnít wrinkle.
To get the rounded top of the legs found on a mini-fig, use a half circle of the 12" cement form tubing you bought. Wrap it in poster board and finish like the rest of the body.
The rear end is just left open as a flap to allow sitting.
This is where you decide how comfortable you want to be and how accurate you want the costume. I chose to do rigid legs that went up to the waist piece. There was a large cutout behind the knees at the top of the calf to allow me to bend my leg and sit down. There was a lot of knocking between the two legs and into the waist piece so it could use improvement.
I cut 3 horseshoe shaped pieces of foam, glued them together, cut a circle that my hand can slide though, and made the wrist out of small diameter cardboard tubing. Again this is somewhere that youíll decide if you sacrifice comfort for accuracy.
One nice thing I found was that you can build the body covering like a coat and get multiple character. The white lab coat on this can lift straight up and off the body to allow changing characters. This is a nice detail if you want to use it more than one year and not start over.
The cost will add up on this costume to do it as described so budget for $150 - $200 between paint, foam, tools, poster board, and yellow clothing underneath. Then there are the hours. Even with all the costumes online, I had to learn a lot of lessons on this one so hopefully it saves you time. Start months early since a lot of steps mean 10 minutes of painting or gluing and a day of drying.
Good luck and Iíll try to watch the comments section if you have questions. HAVE FUN!!!