Where do I input mileage on federal tax return and how do I calculate it into a dollar amount?

They go on Schedule A Itemized Deductions in the Misc section subject to a 2% of Adjusted Gross Income threshold.  If you don't itemize, then deducting them may not benefit you.  

See the below answer to FAQ on where to report job related expenses.

https://ttlc.intuit.com/replies/4800418