In order to book a pool car, navigate to Resources > Pool cars > Bookings > Make a booking.
At the top of the screen, you can choose the dates that you would like to make a booking for. Below, you will find the list of pool cars ranked by their availability. If any overlap exists between your desired booking dates and existing bookings for a vehicle, the overlap will be displayed below the vehicles.

Click the calendar icon that floats on the image to book an available car for an employee.

Alternatively, you can use the relevant Headlight operation in a workflow, to create a booking.