Withdraw food but not water 12-24hrs prior so the gut is empty when you draw them.
what you need is a burco boiler or a big pot, get it near boiling. Kill them by stretching the neck (note it's hold the back of the skull in one hand and stretch away from the other hand holding the legs, not 'wring' as some people say) one at a time when you have plucked the previous one, dunk the chicken for ten seconds in the hot water once it has stopped flapping. Hang the chicken up and pluck immediately, you will have it plucked in <5 mins, probably 2 mins when you get quick. the feathers just fall out. then pick tiny pin feathers (don't touch the skin or it will mark), another 2 mins.
pluck one after another. Line them up on a table or hang them up in a cool place, when all plucked you rest them in the cool for a few hours so fat cools and solidifies. Draw them like a pheasant, neck/crop first, other end next. Finally remove feet and wings.
You can dry pluck, will take you as a novice probably 15 mins each and more risk of tearing skin. i would think a morning and you'll be done. Or find an old boy/retired keeper etc who knows what they are doing and pay them.
A butcher may do it for you, but if you're going to do it regularly, try to do it yourself and learn, it's worth it. Also not sure a plant will take a random few birds for disease risk etc.