First you make a white sauce, one of the first things my Grandma Snarr taught me to cook. She said that if I learned to make a good white sauce, I could make all kinds of other sauces and gravies.
Here's my usual recipe. Just remember the preportions are 1 tbl flour to 1 tbl butter to 1 cup milk, and you can make however little or much you want.
White sauce:
2 tablespoons butter or lipid of your choice
2 tablespoons flour
2 cups milk
Melt butter, stir in flour, remove from heat and gradually stir in milk. Heat heat on medium or medium high, stirring frequently, until mixture starts to bubble, then reduce heat to medium and stir constantly until sauce is thickened to your satisfaction.
If you are making cheese sauce, add one cup grated cheese just before the sauce is thick enough, remove from heat and stir until cheese is melted. If you want to make that two cups, you can, but if you want more than that, make more white sauce.
Season to taste...if using a powdered spice (like 1/4 tsp Coleman's dried mustard), mix it in with the flour so it will dissolve well. You will have no problem with granular spices.
This sauce doesn't take much to fix, except patience at the beginning.