After you have confirmed availability with your tutor, you can book them from their profile:
The tutor will then be sent a booking request, and you will receive a confirmation once they've confirmed your booking.
Under your account section, bookings that are confirmed will have a 'Pay' button. You need to pay for your booking before you can start your lesson. Once you have paid for the session, the blue "Launch Session" button will be displayed.
Click "Pay".