Heat. Just kidding.
Melt for what? Melt over stuff, melt in stuff? I’m one of those instant gratification microwave users, so melting cheese over stuff has never been a problem. Also, the broiler setting in the oven is great if you’re lookin’ for that slightly-browned-n-crispy action on top of things.
Melting cheddar inside stuff is a matter of shredding, stirring, and adding your own emulsifiers, like when you make nacho cheese sauce. (I use cornstarch in small amounts instead of flour but I’m sure there are other thickeners that will work, too.) PS: Bacon or cheese crisps as tortilla chips.
