Charity is personal
A sole proprietor's contribution to a charity is reported on Schedule A as an itemized deduction, subject to AGI limits (60% of AGI for cash contributions to public charities, lower limits for non-cash and certain other categories). The Schedule C business does not deduct the donation.
Sponsorship is business
A "sponsorship" — paying for ad space in a charity program, a banner at a charity event, sponsoring a youth sports team with your business name on the jerseys — is an advertising expense, deductible on Schedule C line 8 ("Advertising"). The primary motive must be business promotion; the charitable benefit is secondary.
Why this matters
The Schedule C deduction reduces both income tax and self-employment tax. The Schedule A charitable deduction reduces only income tax (and only if you itemize). For a sole proprietor, the same dollar spent on a sponsorship vs a charity check can produce 30%-50% more total tax savings as a business expense.
Substantiation
A charitable contribution needs a written acknowledgment from the charity for any donation of $250 or more. A sponsorship needs the same business-expense substantiation as any advertising — invoice from the recipient, evidence of the business promotion (logo placement, ad copy, banner photo), and documented business connection.
Be careful with structure
Don't double-dip. A single $1,000 payment can be either a charitable contribution OR an advertising expense, but not both. Choose the characterization that produces the better tax result and document accordingly.
Where the IRS publishes guidance on this topic
The IRS publishes a layered set of free resources on most small-business tax topics: a relevant publication (usually one of Pub 334, 463, 535, 587, 946, 560, or 583), the instructions to the relevant form, the "Small Business and Self-Employed Tax Center" landing pages on IRS.gov, and the Audit Techniques Guides written for IRS examiners. Reading the agency's own materials is the cheapest tax education available. They are written in plain English, updated annually, and they reflect exactly the framework an examiner will apply if your return is selected.