CX is very straight forward, just get the PNR and off you go, can do it on line.
AA is the same... just bring up the booking and off you go. if you have status with OW, you enter that in to the AA site and it opens up preferred seating.
worth noting AA will link any associated bookings without status (if you are flying in coach) and afford preferred seating to members of the same party. In my case it was separate tickets and AA was happy carry over the status benefits.