It's pretty simple. You have to fit these guidelines:to qualify for S corporation status, the corporation must meet the following requirements:Be a domestic corporationHave only allowable shareholders May be individuals, certain trusts, and estates andMay not be partnerships, corporations or non-resident alien shareholdersHave no more than 100 shareholdersHave only one class of stockNot be an ineligible corporation (i.e. certainu00a0 financial institutions, insurance companies, and domestic internationalu00a0 sales corporations).That's from S CorporationsAssuming your company does qualify (most do), then you would file Form 2553 with the IRS within 75 days of your effective date. That would either be the first of the year or the formation of the company.You can do a late filing under Rev Proc 2013-30 if you missed the 75-day cutoff, but then you have to write a letter.