Step 2: The Head!
First you will need to measure from the top of your head to your chin and round up to the nearest inch. My noggin is about 8.5", so I made it an even 9". This will give you how much you will cut from the form tube. Measure that length on the form tube and starting cutting with the handsaw carefully to get the straightest line possible, I used a rubber band to aid me. After its separated, sand the edge you just cut to smooth it down.
Next your going to cut some styrofoam, this can get very messy so have a vacuum/shop-vac on hand. Take the form tube you have cut off and draw the circle out on the styrofoam. Take two sheets of the styrofoam and cut the circle out slightly bigger to sand down later. Do this one more time to create both the top and bottom of the head. You'll have four 3/4" disks total to work with. Two disks for the top and two for the bottom.
The trickiest part of making the head is making the curved ends. Use the spray glue to combine two disks together. Your going to sand this curve into the styrofoam, so take your time and check your work to make sure the curve is even throughout the disk. This will take awhile and there may be easier ways to do this; there could be a part or product somewhere that fits the form tube and provides the curve needed. I just worked with what I had and it turned out great.
Once the curves are similar to the mini-fig's curve, we need to attach the ends to the form tube. Remember when I told you how spray paint loves styrofoam? Well this is the step that you have been waiting for. We are going to tape over the ends.*See alternative methods below* The main reason is to provide that barrier so the styrofoam doesn't make like a magician and disappear. The tape also secures the foam and makes the head a much more durable. Decide which side you want to be the top and secure it by using one continuous strip of tape across the styrofoam, leaving about an inch on the form tube. Continue this until none of the styrofoam is exposed anymore.
Before we tape up the side your going to squeeze your head through, we're going to need a hole made. Take the 8" bowl you'll use for the face later and center it on the bottom piece of styrofoam then mark its position. Now cut the piece out until you have a styrofoam ring. Don't discard this disk, we'll use it in a bit. To tape the bottom is slightly different. Instead of taping across the gap, wrap it around so attaches to the inside of the head. Do this until none of the styrofoam is exposed.
The stub on the top on the head is made up of three 6" disks from the styrofoam. Lucky for you, the 8" disk you just cut out is already two pieces of foam. Cut the disk down to a 6" diameter, make another 6" disk from the foam and glue to the others and you have the stub. Sand the edges so they are smooth.
Now its time to ventilate your head before you attach the stub. Find the center on the top of your head and use the 2" cup to trace your vent hole. Cut through the tape with the X-acto knife and the styrofoam with the steak knife. Find the center of the stub and do the same. Tape up the stub until no styrofoam is showing, leaving the vent hole uncovered. Use 4 or 5 dabs of the Gorilla glue to attach the stub to the head, aligning the vent holes.
After a few suggestions, I figured it would be a good idea to include them into this instructable. Destructions presented a couple of ideas where you could use a higher density Styrofoam then use a lightweight spackel to fill the gaps for an even finish. Yoyocrazyguy mentioned paper mache, which would solve just about all the issues with Styrofoam as well as keeping things nice and smooth. Teethdoc used a product called DecoArt MagiKote which is made specifically for Styrofoam to harden it into a smooth, paintable surface.
In my opinion, the Magikote would work the best in this application. All you would have to do is brush on a couple of coats of it onto the Styrofoam areas to be painted, let dry overnight, then sand till smooth. One catch is that you would have to make sure the Stryofoam is secured from the inside of the head with either tape, gorilla glue, or a combination of the two.