Butterflying a whole chicken simply means you are removing the sternum and backbone of the bird so it can lay flat. This cuts the cooking time in half, looks fancy on a platter and can be done in three simple steps.
Good kitchen shears
Sharp paring knife
Prepare the bird by removing neck and giblets.
With chicken breast side-down on the cutting board and working from the cavity to the neck, cut the chicken down both sides of the backbone and through the ribs with kitchen shears. Discard the backbone.
With chicken breasts in each hand, open the bird like a book. The breast bones will crack as you expose the cavity. Use your sharp paring knife to cut out the soft triangular breast bone.
Place the bird cut-side down (skin-side up) and flatten it with the heel of your hand; use both hands if you need to apply extra pressure.