Ok, first thing to note: wait for a couple of days. Trust me on this.
The method is the same for any version of 1.1.x and any version of 2.0. You download the large upgrade pack and unzip it on your desktop, then use ftp to load all the files to the correct locations. After that you enter yoursite.com/upgrade.php in your browser address bar and follow the prompts.
Note that 1.1.x themes do not work on 2.0 and some mods for 1.1.x do not have corresponding 2.0 versions. Because of this my preference is to remove custom themes and mods before upgrading.