How to flash your Nokia phone with ease

(note: the phone must be switched off). hold '*', '1','green key for picking calls' and 'Power on' .A guy told me that,after telling hm My nokia phone needs flashing. I tried the code ,the screen was blank for like 30seconds,so i quickly removed my battery before taking it for flashing. He said i should have waited while the screen was blank. A different engineer told me that's what he use.HAS ANYONE USED IT BEFORE