Since we are using weak and diluted acids, copper will not easily dissolve by itself. We can promote its dissociation by applying voltage across the terminals.
Connect you power supply the positive to the positive clip and the negative to the negative clip. Make sure the two pennies are not touching, you should be able to notice bubbles forming on both pennies. (There should be more bubbles on the negative, fully submerged end).
Wait an hour or two. You can move onto the next step when the acid develops a green tint.
This is a great time to clean your job with soap or toothpaste. This gets rid of dirt and oil that will cause the plating to flake or rub off. (Don't touch your job with bare hands after it has been cleaned. Oh you can also use rubbing alcohol.)