How do I apply the standard deduction for business expenses?

If you are self-employed or a sole proprietor, your business expenses are entered on Schedule C.  

See this TurboTax support FAQ for business expenses - https://ttlc.intuit.com/turbotax-support/en-us/help-article/self-employment-taxes/self-employed-expenses-deduct/L37ZS1B8T_US_en_US?uid=m8rtohs4

There is no standard deduction for business expenses.   Enter your expenses on Schedule C and TurboTax will automatically put your standard deduction on Line 12 of your tax return.    

You can choose to itemize your deductions or take the standard deduction, but your business expenses are entered separately - they are not part of your standard or itemized deductions.  

Where do I enter Schedule C?   Enter your self-employment income when you create your business return (Schedule C)

What self-employed expenses can I deduct?  

Where do I enter my self-employment business expenses, like supplies and  vehicle mileage?