To execute this glitch, you need to get to the giant collector arms near Command Post 4 on Mustafar. You also need to enter the invincibility cheat code (see cheats for how to unlock it). First get on one of the collector arms and jump off. You will fall through the lava and you will be able to see the entire level from underneath. You are doing this correctly if you notice that there are tiny embers floating about. After a while, if you are playing as a regular unit, the unit will start to flail his arms. After a longer period of time, you will die(you can also jump through in the lava in between the 2 front doorways at the landing platform).
There is another way to execute this glitch. First make sure you don't have the invincibility cheat activated. Go to any location that is close to the lava. The face away from the lava and throw a grenade on the ground. If you did this correctly, your health bar should be empty, with you flying off the platform and into the lava. Instead of you sinking through with the camera view over the lava it will show you flailing your arms as if you fell off of a high height with no lava.