For pasta 1tblsp of butter and flour per one cup of milk is right. So you'd want 2 of each butter/flour and 2 cups milk. You will toss it together and the pasts will soak up a bit anyways so you don't want it too thick.
Melt the butter, and sprinkle the flour, and stir on low hear for about 30 seconds-1 minute. Just until it turns a golden color and you can smell it. Then pour in the milk and whisk for a few minutes until thick. A pinch of salt and pepper.