1) KTEL buses run between towns in the same prefecture and town in different prefectures. To know more about the same, follow the link below:
Based on your destination, you can click on the prefecture in which your destination is located. You will be directed to the KTEL website of that prefecture and you can view destinations, ticket prices etc. on that website.
2) I visited Greece in April 2017 and until then there was no way to book a ticket online. You will have to buy the ticket from the KTEL ticket office.
3) If you need to change buses, you will have to buy the ticket to final destination from the place where you have a stopover.
Let’s say you have to go to Olympia from Nafplios but there is no direct bus. You would first buy a ticket from Nafplios to Tripoli. You can buy the ticket to Olympia only from Tripoli. You cannot buy the ticket from Tripoli to Olympia from Nafplios.
4) Be present at least half an hour before time or maybe even more in peak season to ensure you get a seat. Sometimes buses take passengers in standing as they did in Santorini, sometimes they do not. When I went to Delphi from Athens, there were limited number of tickets, which I think were equal to number of seats.
5) Sometimes you may have to buy the return ticket as well. There were limited number of buses from Athens to Delphi and vice versa and I was asked a time to pick up for the return bus. I am not aware if this is mandatory or the ticket can be purchased from the destination.
6) Major cities have more than one bus terminal and your connecting bus could be from another bus terminal so be aware of that.
7) Buses can run late. But try to be before time because they might leave a couple of minutes early as well. Try to be 15 minutes early. The bus I had to take from Olympia to Pyrgos was running late. Thankfully the bus from Pyrgos to Kalamata was also running late so I didn’t miss it. The bus to Kardamili from Kalamata left slightly before time.
8) Try not to book the last bus. The bus that I was supposed to take from Pyrgos to Kalamata was the last bus and I was lucky that on this particular day it was running late as well.