According to the Contract Law of People's Republic of China (PRC) and the Civil Code of People's Republic of China (PRC), witnesses should observe the following points when providing loan vouchers:
1. The contents of the certificate are true: the witness shall be responsible for the authenticity of the contents of the certificate and shall not intentionally make false statements.
2. The evidence is legal and valid: the witness must have the relevant rights of the proof, which should be true and complete, and have the valid proof of the witness.
3. No harm: The witness's behavior should not cause undue harm to others. Where the witness intentionally exaggerates personal income, conceals family debts, submits false supporting documents, etc., thus causing losses to the lender, the lender has the right to pursue corresponding civil liabilities from the witness according to law.