What is the preferred way to record travel mileage ?
We can track mileage in QuickBooks Online manually, J-I.
Though, there are a few things we need to set up such as creating a vendor profile for the employee, expense account for mileage, product/service item, and others. You can check out this article for the detailed steps to do this: Track Mileage in QuickBooks Online.
If you need more help in setting up these items, please let me know.