If you want to separate the yolk from the white of an egg, the egg has to be as fresh as possible.
The protective membrane that encloses the yolk weakens with age and breaks more easily, and this can cause problems, because if even one speck of yolk gets into the white, it won't be suitable for whisking. So with eggs as fresh as possible there's much less chance of that happening.
To separate yolks from whites, all you do is hold the egg over one bowl and have another bowl beside it. Crack the egg on the side of the bowl, round about its centre, then, using both hands, break it into two halves, one in each hand. Now slip the yolk back and forth from one half-shell to the other, tilting it as you do so and letting the white trickle down into the bowl while you hang on to the yolk. When there is no white left in the shells, pop the yolk into the other bowl.
If you need to separate more than one egg for a recipe, do it one at a time, placing each white in a cup or small bowl before adding it to the whisking bowl. This means that if an accident occurs with, say, the third egg, and you break the yolk, the other two are safe. One query that often comes up in your letters is what do I do with leftover whites if I'm only using yolks, and vice versa. The good news here is that eggs freeze very well, so pack them in small containers and don't forget to label them with the amount – trying to guess how many egg whites you have is not a good idea.