If Sole Proprietor, deduct contribution Schedule C (Form 1040), Profit or Loss From Business, or Schedule F (Form 1040), profit or Loss From Farming.
If Partnership, deduct contributions on Form 1065, U.S. Return of Partnership Income.
If Corporation, deduct contributions on From 1120, U.S. Corporation Income Tax Return, or Form 1120S, U.S. Income Tax Return for an S Corporation.
Further, after you open solo 401k and are ready to make tax deductible contributions, keep in mind that sole proprietors and partners deduct solo 401k plan contributions fro themselves on line 28 of Form 1040. And if you are a partner, contributions for yourself are shown on the Schedule K-1 (Form 1065), Partner's Share of Income, Deductions, Credits, etc., you receive from the partnership.