I forgot to specify. The selection is to be considered a trader, not investor. I know it needs to be selected together with the 2020 return so it is valid for my 2021 trading income.
Can I e-file and then mail the required statement to IRS or do I need to print and my 2020 taxes with the statement making the selection?
A trader must make the mark-to-market election by the original due date (not including extensions) of the tax return for the year prior to the year for which the election becomes effective. You can make the election by attaching a statement either to your income tax return if filed without an extension or to a timely request for an extension of time to file your return. The statement should include the following information:
That you're making an election under section 475(f);
The first tax year for which the election is effective (that is, the tax year for which a timely election is being made); and
The trade or business for which you're making the election.
Refer to the Instructions for Schedule D (Form 1040 or 1040-SR), Capital Gains and Losses PDF for more information on how to make the mark-to-market election.
After making the election to change to the mark-to-market method of accounting, you must change your method of accounting for securities under Revenue Procedure 2019-43 PDF, Section 24.01. In addition to making the election, you'll also be required to file a Form 3115, Application for Change in Accounting Method PDF. Publication 550 PDF describes the procedures for making an election under the section called "Special Rules for Traders in Securities." Non-filing of the Form 3115 mentioned above won't invalidate a timely and valid election.