Much of cooking relies on the sauce. As mani/sauces are variations of the white sauce called béchamel, it is vital to understand a few simple rules in making the roux - the flour and fat base for the sauce. If you follow the technique here you will create smooth and lump-free sauces every time.
Melt the butter in a pan and then sprinkle in the flour. Stir with a wooden spoon over a low heat to make a paste and just cook the flour a little. Pour in the liquid slowly, stirring all the time to make sure there are no lumps.