Hagar, find an expert breeder from your chosen breed to ask, they will know more about everything, as it pertains to our breed. I am too new to selective breeding to feel you should go by my advice, but since you asked, here is what I do. I have not been doing it long enough to prove if it is a good system!
I "cull" for anything obvious as I see it. A chick can be culled at hatch if it is not thriving. Those are the only ones that I trust myself to cull permanently. I like to see them pop out of the egg picking at anything that looks like it may be food.
I start with all the chicks in the brooder until I can move them outside, I have two grow-out pens (A and B.) All chicks start in A which is for unsexed chicks and pullets. B is for known cockerels and "likely culls." As soon as I determine a chick is a male, or is a female with an undesirable quality (leg color, eye color, wrong comb, etc.) they get moved to B. The male and female culls join the layer flock as soon as they are big enough to fend for themselves. I move them four or five at a time so they will have their own group to hang with. Eventually, I have a pen of "fair pullets" and a pen with "fair cockerels."
My layer flock free ranges during the day and I will lose one or two every couple of months to various predators. I don't mind so much if it is the culls that are taken, plus, if I see something exceptional about one of them as they mature, and what I culled them for is not as bad as I thought, I can always move them back. That makes my lack of confidence in selection a bit more forgiving.
Also, if someone is looking for hens, POL pullets, or cockerels for the pot, I will sell from my layer flock, not as Campines, but as layers.
The serious culling, for top line, bottom line, width of body and head, stance, tail set, and such goes on until I select the best for the breeder pens for spring hatches. Then all the rest go to the layer flock to be chickens for a while. I still watch them to see if anything interesting develops as they mature.
Silver Campines are so hard to find that I have to keep a few more than others may settle on because they are so hard to replace. I have plenty this year (I set over 100 eggs last spring) and selection is easier, but for the past two years, I had minor losses that undermined my whole program because individual birds were so important. I have started over three times!
