Is there any publication that explains the proper way to file a Schedule C as a day trader?

If a day trader is engaged in the business of trading, the day trader files a Schedule C as any other taxpayer engaged in a business that requires the filing of a Schedule C. There is no publication specific to day traders. But see the Form 1040, Schedule D Instructions. The section "Traders in Securities" has information for day traders.

Internal Revenue Code section 475(f) and Revenue Procedure 99-17 apply only to traders who elect to use the mark-to-market method of Accounting.

References:

• Publication 550, Investment Income and Expenses
• Publication 535, Business Expenses
• Form 3115 (PDF), Application for Change in Accounting Method
• Form 1040, Schedule D Instructions
• Internal Revenue Code § 475(f)
• Revenue Procedure 99-17