The process involves boiling down milk to remove about 60% of its water content. A good measure is to start with 2¼ cups of 2% milk and reduce it to a cup.
You can use Skim, 2%, and whole milk, but fattier milk like half and half requires less boiling. Evaporated milk is high-fat, so you may want fattier milk for a close substitute.
A one-to-one substitute of heavy cream also works if you're going for the consistency of evaporated milk, but it's a lot fattier, so you'll get a richer final product.