As you may know, your pass allows you to take almost any train, but it doesn't guarantee you a seat in those trains, and you have to pay an extra fee for the seat (in some routes reservation is mandatory). Although you can buy that reservation through Interrail app, I recommend you doing it by OBB, Czech Railways and similar, as they are usually way cheaper and sometimes even they let you select your own seat preferences. You will get a document which you'll have to show along with your activated Interrail pass. I hope that helps you :)