You'll also need to enable USB Debugging on your phone. that's easily enough done by heading to Settings -> Applications -> Development -> USB Debugging. You'll get a warning message, but just ok that you'll be fine.
Once you've done all the above, the process is as follows:
1. Connect the Phone to the PC in Charge Only mode
2. Open a command prompt (Start -> Run -> CMD, you'll need to run as Administrator)
3. Navigate to the folder you extracted ADB to. In my case, the command would be "cd\Android" (minus the quotes).
4. Type adb devices and press enter. You should then see the serial number of your phone displayed on screen. If you don't check, that USB Debugging is enabled and you're connected in Charge Only Mode.
5. Type adb shell, press enter.
6. Type pm setInstallLocation 2 press enter (it's important you enter this exactly as written as it's case sensitive)
7. Type Exit, then disconnect your phone.
8. PROFIT! :)