Whisk the flour into the skim milk in a small saucepan, then place over medium low heat.
Add the shredded Cheddar cheese, butter 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.
When the sauce thickens, cook 3/4 cup uncooked elbow macaroni in rapidly boiling water. Boil for 8 minutes, until tender and then drain. You will have 1 3/4 cup of cooked macaroni.
After the cheese sauce has simmered for 30 minutes, pour the pasta into a medium bowl.
Gently stir in the cheese sauce, and then pour the mixture into a casserole dish.
Cover and freeze.
Bake the frozen macaroni in a 350 degree oven for 50 to 53 minutes, or until the cheese begins to brown slightly.