Complete and file Form 2553:

--No more than 2 months and 15 days after the beginning of the tax year the election is to take effect, or
--At any time during the tax year preceding the tax year it is to take effect.

But there is “relief for late elections.” See the Form 2553 instructions.

TIGTA just issued an interesting report on S Corporation payroll tax avoidance.

https://www.treasury.gov/tigta/auditreports/2021reports/202130042fr.pdf