An oath or affirmation can be administered as part of another notarial act, such as a jurat, or as a separate notarial act in its own right. Whatever the circumstances, the point of an oath or affirmation is to impress upon your client the importance of truthfulness. To accomplish that, you should lend a sense of ceremony and formality to the act.
Typically, you and your client should raise your right hands, palms forward, though this is not a legal requirement in most states. As an alternative, the right hand can be placed over the heart. You generally have the discretion to use the ceremony and gestures you feel will most compellingly appeal to the conscience of your client.
In administering oaths, you may ask the oath-taker to place one hand on the appropriate religious text with the other hand raised in a pledging gesture. Although the religious element is not included, an affirmation may still involve some ceremony.
To further underline the seriousness of taking an oath or affirmation, you also may wish to tell your client that perjury is a serious criminal offense.