To calculate the fixed monthly payment on a 24 month loan using a 360 day basis:

1. Open a new Excel worksheet and enter the following information in cells A1 to A3:
   - A1: Loan amount
   - A2: Annual interest rate
   - A3: Loan term in months (24)
   
2. In cell A4, enter the following formula:
   
   Formula:

   =PMT(A2/12, A3, A1, 0, 0) 

   This formula uses the PMT function to calculate the fixed monthly payment based on the loan amount, annual interest rate, and loan term in months. The last two arguments (0, 0) are optional and set to 0 to indicate that there are no additional fees or charges.
   
3. To adjust the formula to use a 360 day basis instead of a 365 day basis, you can multiply the interest rate by 12/360 instead of 12/365. Here's the modified formula:
   
   Formula:

   =PMT(A2*12/360, A3, A1, 0, 0) 

   This formula uses the same PMT function as before, but multiplies the annual interest rate by 12/360 to adjust for the bank's 360 day basis.

That's it! You should now have the fixed monthly payment for your 24 month loan based on a 360 day basis.