How do I apply benzoyl peroxide to get rid of cold sores?

Don't. Benzoyl peroxide is not recommended for treatment of cold sores. Cold sores are fluid filled bumps then ulcerations on the lips caused by a herpes virus. There is no cure for the virus just some medications to make you comfortable. You can try some over-the-counter meds or talk to your dentist about anti-viral ointments, homeopathic aids and oral meds that may help to heal them quicker.