I do withhold feed on my candidates because:
1. It gives their crops & hopefully their intestines to clear out before they're cleaned.
2. I'm such a cheapskate I don't like to give them feed they won't have time to convert to meat.
I like to get my candidates in cages to make it easier to grab them on butchering day. So I don't have to cause them stress or me unnecessary exertion chasing them around the yard. I like to do this at nightfall, to make it easier to catch them. I put them in cages where they cannot even reach the dirt or grass through the bars. They are butchered the next day, after 12-18 hours of fasting.
I DO GIVE THEM PLENTY OF WATER TO DRINK. They must have that, right up to the time of their execution.
If you haven't let your birds fast before butchering, you can still do them anyway. Just take extra care cutting around the crop and around the vent, take care scooping out the intestines. If some poop comes out it's okay, just rinse it away with lots & lots of water. I wish you success with your first time processing!