Refrigerate for a few hours to let the flavours develop
You can use a whole roasted chicken. Just de-bone the chicken and shred the meat. I've used store bought rotisserie chicken in a pinch.
You can substitute either British Salad Cream instead of mayonnaise if you prefer. An alternative, if you can stomach it, is Miracle Whip, which my mid-West in-laws go nuts over but frankly I cannot stomach the flavour of.