This was my first attempt of creating anything cosplay. So, as my first cosplay I decide to do Edward Elric for the anime convention, A-kon 25. This Automail is made out of craft foam and it took me around 3 months I believe but, of course I had other obligation that got in the way. However, it's the painting process that took the most time. I'm sure there are different/better ways to do this, but again my first time doing this. 

Materials used for this project:
Craft Foam Sheets
Long Black Sleeve/Glove 
Hot Glue
Measuring Tape/Ruler
Masking Tape
Plastic Bags
Scissors
An Arm Manequin (unless you where crazy enough to burn your own hand like I did >.>)
Pictures of Edward's arm from different angles and/or watch some episodes
School Glue(lots of it)
Paint Brushes
Metal Round Push Pins
Hammer
Gloss Varnish
Scotch Heavy Duty Fasteners Strips
Acrylic Black and Silver Paint
Rust-oleum Metallic Spray Paint
A Heat Source (Example: stove, for big pieces/small candal for little pieces)
Red and Blue Wires
Wire Cutters
 

Youtube Videos to Watch
Each one of these videos are tutorials of how to make armor and they all have their own way of making armor. Although I wasn't making armor, it is still making craft foam look like metal so it helps alot. So check each one and see which one you like best or you can takes parts of each video and combine them like I did. 
Cosplay Construction of Edward's Automail by Goldy Marg  
Craft Foam Armor Tutorial By MissInterpretCosplay
Eva Foam Armor: The Basics by DemonWeasel1990