The easiest way is to go to BlackBerryTools OTA
from your BlackBerry's web browser and download BBLight from there. This is referred to as a "Over The Air" (OTA) installation and doesn't require that you have a computer at all. Once installed, run it, select whatever options you want (make sure to also select the "Activate on Startup" option). To make sure it's activated right away (without requiring a restart), bring up the menu and select "Activate".