You'll enter your mileage for those business trips by clicking on the Federal Taxes tab, then Deductions & Credits, then scroll down to Employment Expenses and select Job Expenses for W-2 Income.
TurboTax will take you thru some preliminary
screens, asking about your job and certain expenses. Then you'll come to
the screen marked Any Vehicle Expenses or
Sales. You'll select "yes" to start the vehicle section,
then just follow the prompts.
After answering some questions about your vehicle, you'll come to a screen that says How do you want to enter your mileage? and you'll select I'll enter the total miles I drove for the year. Then you'll make separate entries for total miles driven for all purposes (you can estimate), then the miles driven for job (that's your mileage for those business trips), then commuting miles (you can estimate). The important entry is your business trip miles, since that's what you want to deduct.
Then you'll proceed thru additional questions and come to a screen where you'll be asked if you want to take "actual" expenses or the standard mileage deduction and you'll choose the latter.
The standard mileage rate is $.54 per mile. Keep in mind that these employee business expenses are a "2% misc" itemized deduction, meaning (1) you have to be able to itemize to claim them, and (2) the total "2% misc" deductions have to exceed 2% of your Adjusted Gross Income before you'll see any deduction.
