Setting up full disk encryption for Ubuntu is pretty straightforward. Enter all the neccessary info during instllation, and it magically works. But if you want to use a USB key not to enter the passphrase each and every time you boot your machine, as you would want to do with, say, a server, things quickly get really complicated.
But be not afraid, for you have found the right place to make it just a little bit easier. Join me, on this journey of frustration, bitter disappointment, and, as is always the case with linux, new knowledge.
Also, I should note, while using sudo -s generally is a bad idea, you might consider doing that, as pretty much all commands here require root access anyway. You'll just be typing five extra characters per command.