If the client only needs me to witness their signature, can I just affix my stamp and signature on the document without any notarial certificate?

No. Witnessing of signatures is not an authorized duty of a notary public, except in correlation with another notarial act. If a customer presents you with a document that has no notarial certificate already printed, you should explain the difference between an acknowledgment and a jurat and ask the customer which one they want you to add. You may then print, type, or stamp the appropriate certificate on the document. If there is no room on the document, you may attach a second page with the appropriate wording. However, if you attach a "loose" certificate, you should notate in the certificate what document the certificate is attached to, to prevent your notarial certificate from being attached to another document.