Plug the Live USB into the PC and then turn it on.
If you aren't greeted with an Ubuntu screen showing "Try Ubuntu" or "Install Ubuntu" click on Troubleshooting Starting a Live USB
Choose "Try Ubuntu", you should get a desktop after a few mins.
If you're stuck at a black screen, click on Black Screen After Starting a Live USB
After the desktop has loaded, start a terminal with Ctrl + Alt + T
You should see a command prompt such as ubuntu@ubuntu:~$