Which part are you unsure about?
Cisco Identity Services Engine Administrator Guide, Release 2.3 - Cisco ISE Licenses [Cisco Identity Services Engine] -…
If you're using Smart Licensing then the job is done in 2 seconds.
If you're using traditional licensing (as opposed to Smart Licensing ... which is cool but doesn't work if your PAN nodes have to communicate via an authenticated web proxy) then
Log into each PAN (Primary and Secondary) and capture the "show udi". Then use the information to create your licenses on the licensing portal (you'll need the PAK files that you purchased from Cisco). Enter all the data into the licensing tool and out pops a file that you load into the PAN GUI.
Smart Licensing is the way to go. There is a bit of pain to set your organisation up for Smart Licensing (that's a whole other discussion). And you can convert traditional licenses into Smart Licenses.
