If your dog does not know a stand, train that first. Stacking is simply adjusting their stand to a proper conformation position for your breed and having them remain in that position while you stand in front of them.

Dog will be positioned in a show stack by handler or dog will free stack on cue. Dog should hold position for 5 seconds with the handler standing in front of the dog as to present the dog for inspection to a judge. (Also accepted, Stand: On cue, dog will go from either a sit or down position to a standing position on four legs.)