On your Calendly Home page, select the event.
Expand When can people book this event?.
Scroll down to Buffers.
To build in extra time before an event, at Before event select the time.
To build in extra time after an event, at After event select the time.
Select Save & Close.
If you add extra time before and after an event, you may severely limit the time you're available for booked event.
Check your scheduling site to make sure your buffer hasn't drastically reduced your availability. Copy the scheduling link for the event type and paste it into your web browser and see the booking page.