Gator, since I use commercial scratch as the treat, I waited until they were about eight weeks old and had had a chance to ingest commercial grit for their gizzards. On page two, you can see the little cup I use - was a measure in Slim Fast or something like that. I don't use my hand since I don't want to teach them to zero in on my hand all the time if I'm doing something else in the coop.
Today, I broke the above rule. Sat down on floor, legs out, flopped a cat harness down and let them get all suspicious and then all curious. I didn't use a clicker since I just want to accustom them to this thing as it gets moved around, flopped in any old way, and then stroked along them somewhere. For this exercise I had on a sweatshirt with a front kangaroo pocket in which there was scratch. I'd fish some out and when one worked up the courage to actually peck the harness, I'd slip just a bit of scratch to it in my hand. This will cut down on the panic when I harness one of them the first time. It will take two people, so I have to wait for a friend. Tomorrow, I'll hang the leash here and there to let them get used to this long thing which swings in the breeze. The goal is to take one out of the run but keep it safe. Lots of people "walk" chickens in harness. It's probably like "walking" a cat. You just meander together or follow the animal, picking it up if necessary to go somewhere else.
Gentle Leader (brand) cat harness is what I have. Has bungee cord style leash.