Simply add a new transaction for the number of miles for a business related drive. To do that, go to the Transactions tab, tap a "+" icon at the upper right corner and select Mileage from the list of options. TaxTouch will automatically calculate your deduction for that drive using the Standard Mileage Rate (if you’ve selected that method in the Tax Details. To change your Tax Details, go to the Accounts tab).
For more information see the IRS web site for Self-Employed individuals.