Writing a formal IOU must be legally binding and refer to the following:

1. The full legal names of the borrower and the lender should be written clearly, and they should have the personal signature, fingerprint or signature of the borrower. For handwritten signatures, copies of ID cards and marriage certificates signed by the borrower are required.

2. The loan amount should be written clearly, including the amount in uppercase and lowercase letters. Write clearly the time period of the loan, including the start and end years of the loan and the clear loan period. The specific year, month and day of repayment should be written clearly. Write clearly the year, month, day, time and payment method for repaying the principal and interest of the loan.

3. The interest on the loan should be written clearly, and there should be clear annual or monthly interest rates, as well as the final total amount of loan interest to be paid (including uppercase and lowercase amounts) and other agreements. The interest rate is up to 4 times the bank's similar loan interest rate. Usury interest rates are not protected by the state.

4. Try to choose the method of bank transfer for payment. You can specify the account number and transfer date of the other party in the IOU. For transfers, you can note the purpose of the loan. The borrower is required to write a separate receipt for the money received.