The demist function at top left of the climate panel is supposed to point the flaps at the screen and put the fan up to an appropriate level according to the temperature of the air coming through, so in theory there's nothing else to do except wait for the screen to clear. On my Webasto-equipped TDI with no air-con/flap issues, that is indeed all I need. My Storm, as with all later TDIs, has the electric element. On the one cold morning I used it on, it was rather disappointing, it took ages to clear the screen fully, even after it became clear enough to drive and the engine warmed up.