Set a pot of water to boil for pasta, then measure out milk to it can start warming up.
In a skillet, melt butter.
Add flour to butter. Stir for a few minutes until well combined and mixture thickens a tad.
Whenever water boils, add pasta. Stir occasionally and drain when ready. Under cook noodles a bit if baking.
While pasta is cooking, add milk to roux. You can add slowly and stir or dump all at once. Don't worry if it seems lumpy. As the milk heats up and you stir it will come together. Let thicken.
If baking start adding cheese before sauce becomes too thick. If eating right away, a thicker sauce coats the noodles better. Add cheese about a third at a time while stirring to allow it to melt and combine.
If your sauce is ready before your noodles, grab a few tortilla chips and try it out. Then stir in your noodles. Serve if eating now. Or add to greased casserole dish and top with more cheese and breadcrumbs. Bake @350 for 30 minutes.