Take a Sauce pan and heat. Put butter and let it melt. Add flour and stir using whisk or wooden ladle. Don't let roux get brown. Remove heat so that heat of roux and milk is same.

Now Slowly heat roux and add milk slowly. Don't put all milk it may get more lumps. Now add salt and pepper as per taste. Add nutmeg powder or grade a little, onion, cloves and bay leaf for flavors. After the sauce is cooked remove the flavoring agents onion, cloves and bay leaf. Bechamel sauce or white sauce is ready. You can add cream for cream sauce, cheese for cheese sauce, mushroom for mushroom sauce.