To use the PMT function to calculate the amount of periodic payment, follow these steps:

1. Determine the interest rate per period and the number of periods. For example, if you have a loan with an interest rate of 5% per year and a term of 3 years, the interest rate per period would be 5%/12 (assuming monthly payments) and the number of periods would be 3*12 (36 months).

2. Determine the present value of the loan. This is the amount of money that you borrowed or the current value of the loan. For example, if you borrowed $10,000, the present value would be $10,000.

3. Determine the future value of the loan. This is the amount of money that you will owe at the end of the loan term, assuming no additional payments are made. For example, if the loan has a term of 36 months and the interest rate is 5% per year, the future value would be $10,000*(1+5%/12)^(36) = $11,528.94.

4. Use the PMT function in Excel or Google Sheets to calculate the periodic payment. The PMT function takes three arguments: the interest rate per period, the number of periods, and the present value of the loan. For example, the formula for the monthly payment in the above example would be =PMT(5%/12, 36, 10000). The result would be -$304.14, indicating that the monthly payment would be $304.14.

Note that the PMT function assumes that payments are made at the end of each period. If payments are made at the beginning of each period, you will need to adjust the formula accordingly. Additionally, the PMT function assumes that payments are made on a regular schedule. If payments are irregular, you will need to use a different formula to calculate the periodic payment.