My pension is paid monthly and is based on the income I had at the time of retirement. How do I show that?

What question are you trying to answer?  You should get a 1099R to enter into your return.  

Enter a 1099R under
Federal Taxes
Wages & Income
Then scroll way down to Retirement Plans and Social Security
Then IRA, 401(k), Pension Plan Withdrawals (1099-R) – Click Start

If it’s asking if you took the RMD,  Say all of it is the RMD.   If you need to take the RMD it will ask you if it is the RMD.  Say yes.  Anything your pension pays you is considered to be the  RMD.  Traditional pensions automatically fulfill the rules of an RMD. So for the RMD question put in the same amount as box 1.

If you receive a pension you should have received a Form 1099-R.  You would report the information on that form on your tax return.  The fact that you are paid monthly and the amount is based on your income at the time of your retirement are not things that you need to report on your tax return.   If you didn't receive a Form 1099-R, you need to contac the pension plan administrator to get a copy.