At a minimum, when requeening, they need to be queenless for 2 hours. I prefer 12 hours. 24 is ok. More and they will have a queen cell of their own started.
No, you can't just take the old one out and put the new one in. They have to want a new one first.
The typical requeening is a candy cage where you pull the cork on the candy end and put the cage somwhere near or above the brood nest where the bees can get access to both the candy and the screen. This works most of the time in most situations. The point Bjorn is making is that some situations are tricker and there are tricks to improve the odds if there are things that reduce the odds such as a hot hive, laying workers, Russian queen in an Italian hive, hive that has already rejected one queen, etc.