At an event: Procedures vary in detail but not in principle. Upon arrival, park as requested and proceed on foot to REGISTRATION, usually several cars parked for the purpose at a strategic part of the car park. Register and pay for a course appropriate to your ability. Enquire how far the start is from this registration point and book a start time which will give time to reach the start several minutes early. Some events operate "Turn up and Go" where you may start at any time that the start is open as long as you are the only one on your course at that time.
The time span shown on event details shows the range of times when registrations can be taken.