[limey]

About twice the volume of flour to fat (hence a level tbsp fat and a rounded tbsp flour above), this is about the right amount for half-a-pint of milk, but experiment.
NEVER try to add more flour on its own once you've started - that is the route to guaranteed lumps.
Cooking something in the fat and then adding the flour, as above, as pretty foolproof.
If you're after a classic white sauce, melt the fat, add twice the volume of flour and mix over heat until it forms a soft ball. Gradually add the milk as above, stirring and beating it in until it is completely smooth before adding more.
If you get lumps make sure you've got a balloon whisk (wire whisk) handy and beat the shit out of the sauce when no-one's looking.
And finally ...
it's always edible, even if it doesn't look it!