But how do you not get attached to an animal you've had since it was a wobbly little fluffball? Or does attachment not matter so much if you have more of that practice?
I think it varies a bit from one person to another. Yes, practice does help there too.
A few ideas, in no particular order:
It can help to know from the beginning that you will butcher some of them. (Example: purpose-raised meat chickens, or buy 4 cockerels and plan to keep one for breeding, then spend the whole growing period looking at them for reasons to eat this one or that one.)
Having more of them at a time can help. Having birds that look alike can help. It is much easier to become attached to the white one and the brown one and the black one, than to become attached to every one of twenty same-color ones.
I remember the explanation my mother gave when we first got rabbits: these cute little bunnies will be pets. When they grow up, they will have bunnies, and we will eat those. We kids had plenty of advance warning, we had an adult making sure it did happen, and I think it did help that we knew certain ones were going to stick around. (We did not keep any of them into old age, but after a few years it was not such a big deal to butcher the formerly-pet ones and replace them with new breeders chosen from a recent litter.)
I regularly see complaints about people who get an animal (example: puppy), and when it grows up they get tired of it, get rid of it, and get another young one. That is considered bad when it happens with dogs that are kept as pets. But an attitude like that can be helpful if you are dealing with animals that you DO intend to eat. Focusing on the new fun ones can make it easier to butcher the not-as-new ones.
If you want to practice killing & eating a chicken that you did not raise yourself, maybe consider buying one or more live chickens for that purpose. (Some people need to get rid of old hens or extra cockerels, don't mind them being eaten, but don't want to do it themselves.)