Is a customer asking for their tickets, or do you want to print them yourself? You open them straight from the order.
Open your event and go to Orders.
Search for the order by name, email or reference and click it.
At the top right, click Open tickets.
The customer's ticket page opens in a new tab. There you can save the tickets as a PDF or print them.
Would you rather email the tickets to the customer again? Read Resend tickets to a customer.