Your estimated tax for each payment period is due 15 days after the end of the period. For example, for income that you received from June 1 to August 31, the estimated tax is due September 15. So when you know how much income you received in the period, you have 15 days to make an estimated tax payment based on that amount of income.
To keep it simple, you could just pay a fixed percentage of the income you received. You could calculate the percentage based on your total tax as a percentage of your total income for the previous year. Don't forget to include self-employment tax.
