Just to add to the above - not sure of your level of knowledge, but keeping it simple it's a bit like using SMS or text messaging.
You normally have the option of displaying a CPDLC page on one of the flight deck displays.
Logon is usually done by simply entering the 4 letter centre code on a log in page and selecting "send". FWIW many places expect you to log in at least 15 minutes before entering their airspace unless it's an automated handover (see below), though as you leave VHF cover you may get a final reminder e.g." continue with Shannon now on CPDLC". FWIW there's usually only one code per centre and they're in our documentation, so in many busy parts of the world ATC won't read out the 4 letters by voice unless asked.
Having sent the log in there then usually follows text confirmation from the ground that it has worked. You usually also have a HF or sometimes VHF backup.
As you cross from one CPDLC equipped centre to another CPDLC equipped centre (e.g. Mid Atlantic from Shannon to Gander) the switch between the two is automatic but you do get a "text" confirmation and usually also there's a requirement to do a check in by voice on VHF or HF with the new centre.
Once you leave a CPDLC area and enter a non CPDLC area the system usually automatically logs off ( again you get confirmatory "text message"). You also always have the option at any time to log off manually from the flight deck.
Last edited by wiggy; 27th Nov 2016 at 08:22.