Simply heat up a skillet or frying pan, spread butter or margarine on the bread, place one piece of bread margarine side down in the skillet, a slice or two of cheese, typically a processed cheese-food slice or Cheddar cheese and the other piece of bread, greasy-side up.
If you have a bacon press put it on top. When the bottom slice has turned brown, flip the sandwich over and put the bacon press on it if you have one.
The goal is to get a nicely browned, sandwich with melted cheese in the middle.An example recipe