Since my updated post on building a P25 trunking scanner with a Raspberry Pi and an RTL-SDR dongle, I’ve created a copy-and-play image file that gets you up and running in minutes.
I’ve put it (along with a couple of other RPi image files) at http://febo.com/pages/os_images/. The filenames are somewhat long, but modestly descriptive so you should be able to figure out which to download.
Once you have the file, unzip it and copy to an 8GB SD card. Put that in an RPi 3, attach an RTL-SDR.com v3, and adjust the configuration files to your liking, and you’re on the air.