I normally do a self check-in to reserve my seats (no, I have not done so on Qatar Airways).
Normally, the self check-in is available 24 hrs prior to the scheduled flight time. IF you need a special seat (it is indicated on the seat plan), then you have to pay. If not, the other available seats, you can reserve without any charge.
If you are a privileged member, maybe you can do it earlier than 24hrs.
Of course you will need the Booking Reference (PNR number) OR eTicket No and Your last name to do this.
Booking Reference (or PNR number) consists of 6 numbers and letters, e.g. YBIRI7, and the letters are always written in BLOCK LETTERS.
https://cki.qatarairways.com/cki/dashboard
I hope this helps.