The rule is simple: if your total tax liability for the year, after TDS, exceeds ₹10,000, you're required to pay it in instalments during the year rather than as one lumpsum after year-end. The schedule is 15% by June 15, a cumulative 45% by September 15, 75% by December 15, and the full 100% by March 15.
This applies to freelance income, rental income, capital gains, interest income, and any other income where tax isn't already being deducted at source in full. Missing these deadlines doesn't mean you can't pay later — it means you pay interest under Sections 234B and 234C on the shortfall, which adds up if ignored for a full year.
The practical approach: estimate your total income for the year as early as possible, calculate the expected tax, and set calendar reminders for each instalment date — the Financial Calendar page on this site has all four dates listed together for exactly this purpose.