Starting on a PC using the booking system in MS Access. We book two seats in the middle of the auditorium.
From there we print the ticket that will be sent by email (as a PDF) or by post.
On each ticket is a QR code that can be scanned to show which seats are booked.
And you can see where you're sitting if you scan the QR code with a phone or tablet. Of course you don't need to do this because the ticket itself is sufficient.