I'm glad I got back to this site, because this is my problem, too! I have never had to cull; it always happened naturally--old age, predators, sharing with a friend. But now I have 29 3-yr old mixed breed hens, who are rarely laying. I also have 10 new pullets who will start laying in a month (but I think one is a roo!), so my coop is too crowded. What I did, was ask a friend if she knew of someone who wanted older hens. I put zip-ties on the legs of the few girls left who seemed to be laying. My friend brought someone to my coop who wanted 6 of the hens (to eat). I had intended to charge, but he really needed them for food--not much income.
Finally sharing "my" solution. I told friend not to pick zip tied girls or young buffs. Then I left. I couldn't pick which ones had to DIE! It worked well for me.
But NOW I need to go to another thread, because the last three or four days, I have only gotten ONE egg!!!