you must estimate your total tax for the year.,
you can base your estimate on prior year's tax , or 90% of this year's tax, whichever is smaller.
each period, your estimated tax paid and withheld must be at least 25% of the estimate, even if your income is uneven, payments above 25% are carried into the next period.
if your estimate is based on this year's tax and turns out to be wrong you may be penalized.
you can compensate by overestimating.
if your estimate is based on prior year's tax, you know that when you file by April 15, which is also the first estimated tax payment due date. How convenient.
@orchid9
