I did this recently and was dead easy
Just plug the blue ethernet cable into the modem and pc. Open up your browswer and go to
https://autoreg.autoregister.net You'l need your pin number, serial number to use is ntl-ct8-cj486
Follow the registration and you'll need to set up a new email address. Once registered the email addresses from the STB account can be transferred onto the SACM account.
Modem is quite smallish, he will connect it to the existing conx or wherever you want it. Easy peasy, I was up and running in under 5 minutes (and that was with reconnecting my router too!!)