Form 2553
The form you send the IRS to have a corporation or LLC taxed as an S corporation.
Want your corporation or eligible LLC taxed as an S corp? You file Form 2553, Election by a Small Business Corporation. The rules are in the Form 2553 instructions.
Timing matters. File any time in the year before, or no later than 2 months and 15 days into the tax year you want it to start. A calendar-year company aiming at 2027 has until March 15, 2027. Miss that and the form usually counts for the following year.
One officer signs the election itself. Then everyone holding stock on the day it's made signs a consent. If you're an LLC, this form is enough. You don't file Form 8832 on top.
Send it to the IRS by mail or fax. Acceptance comes back as a CP261 notice. Missed the deadline? Rev. Proc. 2013-30 gives relief for up to 3 years and 75 days if you have reasonable cause and write "FILED PURSUANT TO REV. PROC. 2013-30" across the top.
In New York and New Jersey there's a separate state election to make too.