Lay cheese slices on 1 bread slice. Top with the other bread slice, buttered side up. (The buttered sides face out so that they'll turn crisp and golden when they touch the hot pan.)
Heat a medium frying pan over medium heat. Flick a drop of water into the pan; if it sizzles and evaporates, the pan is hot enough.
Put sandwich, buttered side down, in pan with both hands. Let it cook 3 to 4 minutes, or until golden and crunchy underneath. Using a spatula (or one in each hand, if the bread is large), turn the sandwich over. Let it cook another 3 to 4 minutes, or until golden and crunchy on the second side.
*Choose a cheese that melts well. Some classics for grilled cheese: cheddar, jack and pepper jack, Colby, gruyère, Swiss, fontina, provolone. Brie works well too.