If you want to make some from scratch, I guess it depends on your definition of scrach, but here is the basic of my nacho cheese covering. I call it covering because I like to layer a cover on top of the chips I lay on a plate.
First, you need a big can of nacho cheese. I can't remember the brand I get, but it is blue with pictures of cheese and chiles and is a lot. It may be close to 1 gallon. I have to squeeze it into the vegetable tray sideways.
Fill a large pan half way with water.
Fill a smaller pan with as much nacho cheese as you want to deal with.
Place smaller pan inside larger pan and use it as a sort of double boiler.
The boiling water in the large pan melts the cheese and the second smaller pans is kept from burning. Sometimes I skip the double boiling bit, but if you aren't attentive you'll burn your cheese.
That is the basics. However, depending on what I have on hand, the following additions are made:
Jalapenos, sliced (banana pepper can substitute if really necessary but just don't quite feel right).
Sometimes the jalapeno juice.
Remember the seeds and veins are where the hot peppers get their heat from.
Diced tomatoes. Strain out the juice if you use them from the can (and I almost always do if I add tomatoes, just because it is such a pain with fresh tomatoes, but probably tastes way better).
Black olives. Yeah, black olives are plain and don't have a "taste" (compared to green or what have you) but they are a great addition to nachos.
Other cheese. Maybe you want real cheese, so grate some up. Maybe you want to add a can of Campbells cheese soup, or some left over fancy stuff from a party. Whatever, just remember to make the pieces small enough to melt evenly with the rest of the sauce.
How long to cook? Long enough to make it melty enough to pour or spread to your taste.
Personally I like it thicker, but I used to have a friend who'd add milk to thin it out. Never did like that.