Patients can give their consent expressly (either verbally or in writing) or it can be implied (by words, behaviour or circumstances).
Having a patient sign a form does not ensure they understand. You need to take time to provide all the information they need to make a decision and answer any questions they may have.
Check in with them often. Even if a patient initially consents, they can refuse treatment or change their mind and withdraw their consent at any time.
Document in the patient's chart that consent was obtained. Include information such as: the information discussed, who was involved in the discussion, and the questions asked and answered. If you do have the patient sign a written consent form, be sure to include it in the patient's chart.