You can assign drivers manually by going to the Scheduler page and click on the "Driver" tab in the upper left (below the calendar).
Find your driver, click and drag their name to your chosen vehicle.
Depending on where you drop the driver, it will create a default shift. You can drag the top and the bottom of the shift to change the shift start times and end times.
You can also click on the Shift times and type in a start and end time manually.