That is the exact same pattern that I am going to be using to make my own Automail Arm. But instead of her Plastic sheeting and foam technique used in the tutorial, I will be using aluminum sheeting (only $17.99 for a 100' roll at Home Depot). Due to the fact that it is silver to begin with, that will save time worrying about the paint job adhering to the plastic, or even how it might scrape away with use. Any scuffs can be polished out, or left in to give it the look of being used the way Ed abuses his automail.
I also suggest looking at the first link on that page, to janchan's site. He has some excellent modifications to the pattern, as well as some high-res images of the finished product, plus a pattern for Ed's coat.
Good luck in the construction, and take your time doing it. The worst thing you can do for a project like this is to rush it. Plus for it being your first cosplay, you want to do as much as you can to make it look the part. Even if all you have time for is the arm, you can always nix the coat for your cosplay and go wearing a black shirt, showing off your automail.
Can there ever be rest for those such as I?
SOUL LARP 2010: Nyanne - Male Mithra Dragoon
Sun: Edward Elric (FMA)
Shugo - .hack//