The heat gun is.. well.. a heat gun. It's got two settings, hot and hotter.
Caution: Heat guns get HOT! You can easily burn yourself on the exposed metal, the hot air, or on whatever you're heating up. Be really careful.
Plug it in. It has three settings, Off, Low, and High. You make the right choice.
Make sure it's cooled before you put it away or the tip may melt things it contacts!