Using a tool like the one in the workshop manual bored to correct ID and tapered on the OD works fine.
Warming the teflon seals up and slipping them on the mandrel works fine. Then while still warm slip the mandrel onto the valve and slip the rings into place one at a time. Just letting them cool contracted into place. No need to press or force them. Just take it slow putting the valve back in the housing so they aren't damaged.
That's a good idea , using the housing to size them . You say this is in the workshop manual ? When I go back to work I'm going to check that out . I don't ever remember seeing that stuff . Was it called the overhaul manual by any chance ? That's a whole different book . You should look around for the old Operation Pride book of TSB's . I think you would like that . It's really the best thing (maybe only good thing) BMW did while they owned Rover. It was about as thick as a smaller phone book , loaded with fixes for a lot of common shittery.