Explain Gauss to me like I’m an idiot

If someone could explain the best way to use Gauss’ Thermal Sunder (the fire or cold aoe attack) I’d really appreciate it. Explain it like I’m some kind of hypothetical idiot who doesn’t get the guides on YouTube. What’s the best way to get damage out of it?

Literally just tell me what buttons to press if you have to lmao. Thanks