I just held the chick butt under a slow running stream of warm water and let the poo soften and worked it off. Don't pull it or you could pull out down and tissue from their vent. Just work it between your thumb and fore finger untill it falls away. It takes just a minute.
This site is the best!!! I'm new to raising chickens and one of my three day old chicks had a poopy butt and I didn't know what to do about it. Found my answer here in no time at all, and found I wasn't the only one to have this happen. Thanks everyone for the great answers to my not so unique...