Ok, here is how I make cheese sauce.
1. Add some butter to a pan over medium heat, so it melts. Stir well, so it does not seperate (foam on top is bad)
2. Now turn down the heat a bit, add an equal amount of flour, and whisk well. This creates a nice blonde rouix (sp?)
3. Slowly add milk to the rouix, whisking constantly. If you add too much milk at a time, you get lots of lumps that are nearly impossible to remove. (I add liek 1-2 Tbsp at a time, until the mixutre is very creamy)
4. You now have a nice base for melting in cheese. You can make a thinner sauce (for soup bases, etc) or you can overload the cheese for something like mac n cheese.
I usually just eyeball it, so I cannot give you good measurements. Bu as a guess:
4 Tbsp butter
4 Tbsp flour
1 cp milke
1-3 cups cheese.
Like all cooking, it takes practice. Your sauce will likely be lumpy the first try. After the second or third batch, you can make some really excellent (and simple) cheese sauce.
EDIT:
Please note that you cannot simply "melt" cheese into a nice sauce. That is some idea that the velveeta people came up with. But we all know that velveeta is not cheese.