How to pay Income-Tax ? - for a RETIRED person?

Go to a good CPA, they will estimate your tax due and can set you up with payment vouchers to make estimated tax payments. Otherwise save the money over the year in a savings account and earn some interest and then pay when you file income tax return. They will not give you a break for paying your tax early so its only worth it to avoid any penalty you might incur. Everything depends on a lot of things regarding your specific situation. Tax advice is complicated.