Estimate quarterly estimated taxes.
Quarterly estimates exist because self-employed income has no withholding. Adding the income tax rate to the self-employment or social contribution rate gives a combined rate applied to expected annual profit; a quarter of that is each instalment. Subtracting what you have already paid shows the balance, and turning it into a monthly transfer is what makes the next payment date uneventful.
Quarterly estimated tax
Annual tax = expected profit x (income tax rate + self-employment rate)/100; quarterly payment = annual tax / 4
Estimated tax rules, rates, thresholds and due dates differ by jurisdiction and change frequently. Treat this as a planning estimate only and confirm your obligations with a qualified tax professional.
Most tax authorities charge interest or an underpayment penalty even if the annual return is settled in full, which is why estimates are worth getting roughly right rather than ignoring.
Some jurisdictions allow an annualised income method that matches instalments to when income was earned. It requires more record keeping but avoids overpaying early in the year.
Profit. Applying rates to revenue substantially overstates the liability because business expenses are deductible.