If I open a SEP ira, and am self employed as a sole proprietor, how can I make contributions as the “employer” so they are business expenses and reduce SE tax?

You can't. Your SEP contribution has to be calculated after your tax return is complete in all other respects.

A SEP contribution by a sole proprietor is not a business expense, it's an individual above-the-line deduction.  As fanfare said, it cannot be used to reduce self-employment taxes.  It only reduces AGI.  Only SEP contributions for your employees (other than yourself) are a business expense that is deductible on Schedule C.