We had a hen with an impacted crop and this is what we did:
My husband held her, wrapped in a clean, dry towel. My mom, a nurse practioner, sliced into the crop, very gently, with a sterilized Xacto knife. I was asked to leave the room and sit outside with my head down. Mom pulled out grass clippings, undigested corn and a good half cup of other things while this hen calmly rested in Bobs arms. There was so much removed that the cut skin really didn't need to be stitched up. Just closed up on it's own.
We kept her separated for a few days, giving her the regular layer, and then she rejoined the group like she had been on vacation, just bounding with energy. That was back in April or May and she's fine still.
Gail Damerow gives better instructions in The Chicken Health Handbook, p 264, with a handy diagram on p46.