Wonderful question!
The Mark-to-Market Election
Traders can choose to use the mark-to-market rules, investors can't. If a trader doesn't make a valid mark-to-market election under section 475(f), then he or she must treat the gains and losses from sales of securities as capital gains and losses and report the sales on Schedule D (Form 1040 or 1040-SR), Capital Gains and Losses (PDF) and on Form 8949, Sales and Other Dispositions of Capital Assets (PDF) as appropriate. When reporting on Schedule D, both the limitations on capital losses and the wash sales rules continue to apply.
After declaring yourself as a securities trader (if you objectively meet the criteria) you will then need to print and file your tax return on paper (not e-file) and attach a statement to the back of it declaring what is known as an IRC (Internal Revenue Code) Section 475(f) election.
If you properly made the mark-to-market election with the IRS, you should report all gains and losses from trading as ordinary gains and losses on Part II of Form 4797, instead of as capital gains and losses on Schedule D. In that case, securities held at the end of the year in your business as a trader are marked to market by treating them as if they were sold (and re-acquired) for fair market value on the last business day of the year.
You will manually add a Form 4797 to your tax return, and enter your net total gain / loss in Part II of Form 4797, using the TurboTax smart worksheet.
Please note that you will want the TurboTax desktop software to correctly do this; the TurboTax online program will not be adequate, because you will not have access to the "Forms Mode" of the software.
Traders report their business expenses on Schedule C. Commissions and other costs of acquiring or disposing of securities aren't deductible but must be used to figure gain or loss upon disposition of the securities.
Of course, the TTD 2021 version is not yet available, but the forms for the current year if you'd like to review what any of them look like are public from the IRS for the current year (so I took care to note form numbers above).
The Journal of Accountancy has a nice breakdown on the topic as well:
https://www.journalofaccountancy.com/issues/2014/jul/sec-475-election-20149537.html
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