Mac and Cheese...Help Needed!

This is one of those recipes I just cannot seem to get to work right, so I need your help, readers!  I'll give you the run down of what I did...do me a favor and read through, and please help me out with any tips you may have, or tricks you use (if you are willing to share them)!  The taste this time was much better than the one I made before, but this time, the consistency of the sauce was TOTALLY off!  I might have to give up on making mac and cheese soon if I don't start getting it right!

Mac and Cheese
Yields 6 generous servings

To Make the  Mac and Cheese
1. Bring 3 cups of whole milk to a boil in a large pan.

2. Add 2 tablespoons of cornstarch or flour to the milk and blend well.
3. Add 24 ounces of shredded Monterey Jack and Colby mix cheese.  Add the cheese in small quantities, making sure to blend it into the milk until it is melted and fully incorporated into the sauce.

5. Once all of the cheese is mixed into the milk, add uncooked elbow macaroni.  I have tried doing the macaroni both ways...cooked and uncooked...but I think the sauce holds up a little better when I add cooked macaroni.

6. Heat the oven to 350.
7. Put the mac and cheese in a casserole dish and top with 8 more ounces of shredded cheese.

 8. Bake until the pasta is tender and the top of the casserole is golden brown and melted.