Viajero,

1) Buy ticket. Date the first segment, leave the rest open
2) Make reservation for the open segments through your travel agent or the airline
3) When reservation is made, airline will at times ask for a ticket number
4) Give it to them if they do
5) Fly with a big smile on your face

They ask for the ticket number to make sure that the reservation is at least somewhat legitimate. There is the occasional problem of agents basically holding seats open by reserving them and later releasing them.