I felt that Mark's recipe for white sauce was missing some of the more important details, so here's the approach I took the first time I made white sauce...

Put the flour and oil into a saucepan, and stir until evenly mixed. Stir some more until all the lumps are gone. Realise this is never going to happen, and just stir until you can't see the lumps any more. Put on the hob and wonder how hot it should be, then change your mind and turn it down in case it burns. Stir some more, and clean up the proto-sauce from the hob. Peer at it and wonder why it isn't doing anything, wonder what "sizzle" is meant to mean, and turn the heat up a bit. Go away and add herbs to the bolognese, forget about the white sauce, then come back to find it foaming and filling half the saucepan. Stir vigorously until it's calmed down a bit. Decide that it's probably gone through the "sizzle" stage by now and start adding the milk. Stare in a horrified way at the huge lumps of dough-like stuff that have suddenly appeared in the sauce. Turn the heat down, add more milk and stir some more, spilling sauce all over the hob. Clean the hob. Add the rest of the milk a bit at a time, stirring it in. Add the cheese for good luck, since that's what Mark normally does. Note that what you have now appears to be warm, cheesy milk, and worry a bit. Stir for another five minutes. Go back and read the original recipe, and realise that you missed "bring to the boil slowly". Turn the heat up a bit. Decide that it's clearly a failure, that tonight's lasagne will probably resemble soup, and that your cooking skills generally suck. Be reassured by Ramsay that Mark normally says it's not going to work around this stage, and stir hopefully for a few more minutes. Note that you still only appear to have warm milk. Go and stir the bolognese for a while and lay out lasagne sheets. Return in desperation to the white sauce to note that it appears to have thickened up nicely. Add to lasagne.