I'm no chef, so look up a book if you want proper instructions, but here's how tondo things:

1. Mix equal parts (by weight) flour and butter in a saucepan over medium heat - this is your roux, and it'll be a bit clumpy
2. Add milk, very slowly, and mix thoroughly after each addition, consider adding nutmeg - this is a béchamel sauce
3. Add grated cheese (and sodium citrate if desired and if your cheeses won't melt well), cut the heat, and mix until smooth - and this is a mornay sauce
4. Pour over pasta and enjoy!

It's not that hard and the result will be better than anything you get from the store or eat at your aunt's house.