Step 14: Try it on and have fun!
Stay hydrated! This is VERY important! Your body is more or less surrounded by 1" or more of foam and you're going to get incredibly hot in it. If you haven't built it to allow a hydration pack to be worn under it, be sure you drink fluids in a more conventional fashion as frequently as you can. Hey, there's entertainment to be had in a monster downing a bottle of water.
Wear something underneath the foam. I wore a pair of bike shorts and a race top. Not only does this prevent friction on sensitive bits, but if you have to exit the costume in public for any reason you won't find yourself parading around in the nude.
Have at least one friend around who is willing to be your wrangler. More is even better. They will be able to watch your back and help you when you need it. This type of costume limits your visibility and your ability to do simple tasks like opening a door. Your wrangler(s) will prevent you from getting into situations that would lead to you getting hurt or your costume getting damaged. People can be complete idiots around somebody in a costume that puts them at a disadvantage and your wranglers will be enough to deter most of the stupidity.
I hope this tutorial has given you ideas! Go make monsters and have some fun!