How to add session data

This may sound like a ridiculous question, but I'm trying to work out how to add data to the session (to indicate a user has logged in). The documentation here says:

You can simply assign data to the 

Code:

$_SESSION

 array, as with any other variable. Or as a property of 

Code:

$this->session

.

How do you add it as a property of $this->session? I can't seem to find a relevant method anywhere and everything online seems to point to set_userdata(), but apparently, that's the 'old' method. I don't want to use the 'old' method, I want to use the new one!