We're going to learn how to make a sphere out of insulation foam (extruded polystyrene). What you do with that sphere is your own business.
Step 1: Materials 'n Such
You're going to need:
Insulation foam - found at you local home supply store. I used 4'x8'x2" sheets. around 4 of them for this sphere. (sorry i don't know exactly how many, i was also working on another project simultaneously
Spray Adhesive - Make sure it's compatible with foam. I recommend Loctite High Performance Spray Adhesive , that stuff worked great, I was surprised how well it worked.
Cutting Tools - Sure, you could use a serrated bread knife, in fact that will come in handy later, but for the major cutting, do yourself a favour and buy a hand Jigsaw! I spent way too much effort cutting out large circles with a knife when I realized that tools had been invented to make life easier, and it did. My jigsaw cost about 30 bucks, and it was so worth it.
Large Chalkboard Compass - I found mine at a school supplies shop. this was a real life-saver. I first tried a string on a nail, but it was inaccurate and a real pain to use. The large compass made drawing large circles a breeze. I know, imagine that, tools made for our convenience.
Shaving/Sanding tools - get a hand rasp like the one pictured, It was the most efficient and least blister inducing. also get a couple different grades of sand-paper, coarse and fine. Use smaller electric sanders if you like.
Sharpies - Sharpies are the best.
Tunes! - get some good tunes goin! ask for me for recommendations, go ahead.