To add your past mileage, you can use the "Add a trip manually" button in the menu of your app.
If you have a lot of previous miles to enter and would like to enter the miles in one bulk amount, no problem! You can go to the menu and click "Import past mileage log." You can add in a bulk number of miles that you drove before using Stride.
Just be sure you are only adding in mileage and expenses for which you have proper documentation so that you can back up the deductions!
Note for all users:
If you have incomplete records or forgot to keep track of your miles throughout the year, have a look at this article for more information on properly adding past mileage, What to do if you didn't track your mileage.