The answer to your question depends upon the structure of the new business. If it is a sole-proprietorship or Single Member LLC, you would simply file the business income and expenses on a separate Schedule C when preparing your individual income tax return for the year. If it is a partnership, C-Corporation, S-Corporation, or Multi-Member LLC, you would file on the appropriate tax form for those entities.
Until the business is producing a net profit from operations, there is no reason to file estimated taxes as taxes are based on showing a net profit. If you have no net profit through the end of August, then no estimated tax payment would be do in September. If you show a net profit through the end of December, then you would want to make an estimated payment by January 15, 2024.
Income or loss from your other entities will effect the amount of tax due as they will be aggregated on your return. Please click on this link as it will help you with determining the amount, if any, that needs to be paid. Estimated Taxes: How to Determine What to Pay and When
