Whisk the flour into the skim milk in a small saucepan then place it over the medium low heat.
Add the shredded cheddar cheese, margarine, and salt and stir often with a spoon until the cheese begins to melt. Reduce the heat to low and simmer for 30 minutes. Use a whisk to stir the sauce every couple of minutes so that it becomes smooth and thick.
While the sauce thickens. Prepare the macaroni by dumping 3/4 cup uncooked elbow macaroni into rapidly boiling water. Boil for 8 minutes or until tender and then strain. You should have about 1 3/4 cups of cooked macaroni.
When the cheese sauce has simmered for 30 minutes, pour the pasta into a medium bowl. Gently stir in the cheese sauce and then pour mixture into a loaf pan or casserole dish. Cover and freeze.