I've got two methods, the fast one and the good one. The fast one--I always just eyeball this, I've got no recipe--is to heat some milk on the stove, then add a bit of butter, a couple of tablespoons flour and some strong-tasting cheese (I like sharp cheddar--mild cheeses' flavors don't hold up). Stir until it thickens, pour over cooked macaroni, and mix in.
The good one is as follows (adapted from Ellen Buchman Ewald's _Recipes for a Small Planet_ (1973), a great vegetarian cookbook if you can find one used):
1 (and a little more) cup dry macaroni, 1 cup bread crumbs (they don't have to be stale; I like wheat or rye), 1 1/2 cups hot milk (skim is fine), 3 beaten eggs, 1/2 cup chopped onion (or less, if you're not an onion freak like I am); 1 1/2 cups grated cheese; 1/2 tsp salt; pinch of cayenne; herbs to taste (the recipe suggests oregano, basil, parslet, rosemary)
Preheat oven to 325. Cook noodles in salted water, drain, and set aside. While you cook noodles, soak bread crumbs in milk. When bread is mushy, stir in eggs, onion, cheese, and seasonings.
Put the noodles in a 1 1/2 quart baking dish. Pour milk mixture over this but don't stir it in. Set casserole in a pan of hot water and bake for an hour, or until it's firm.