As sole proprietor where on Schedule C do I enter charitable contributions?

Generally charitable contributions are not eligible as a business expense on the Schedule C, but if you made a payment to a charitable organization that was not for charity nor a gift, you can choose to deduct the payment as an expense to the business.

For example, if your business benefited from the "donation" such as marketing or etc,  you may deduct the payment as an expense. 

Your other option is to determine if the contribution can be deducted on the Schedule A if you itemize your deductions for your personal tax return.

See the IRS link below that explains more about business expenses: Business Expenses