A beauty mark piercing is relatively simple - the amount of pain is different for everyone. Those with thicker lips will have more flesh to pierce through. Keep in mind that this type of piercing needs both facial and oral aftercare to keep both sides of the jewelry clean at all times. Sea salt soaks (external and internal) are highly recommended.
Typical jewelry for this type of piercing is a labret stud, which is a post jewel that screws into a flat disc-shaped base. The disc lays snug against the inside of your lip, holding the post in place. If the disc is allowed to rub against the teeth or gums, however, it can cause serious irreversible damage. If you see any signs of enamel erosion or gum recession, remove the piercing before it's too late.
The most common problem with disc-back jewelry is skin trying to grow around and envelop the disc base. Don't allow this to happen because if it does, the only way you'll be able to remove it will be to cut it out of your skin. It can also trap bacteria inside with it and lead to infection. The way to avoid this problem is to make sure your jewelry is the right size and that you're not irritating it with unnecessary movement and/or friction. Sea salt soaks will help to both prevent and treat this problem. A change of jewelry to a different size or material is usually the next course of action if the sea salt soaks aren't effective. This should only be done by a professional; changing the jewelry out improperly could lead to even more damage.