The easiest way to install the smart mirror is via the 1-line installation script for the Pi 2 and 3. This script will install the smart mirror and it's dependencies:
curl -sL https://raw.githubusercontent.com/evancohen/smart-mirror/master/scripts/pi-install.sh | bash
Once that script finishes running (~15 minutes on a standard connection) you'll want to configure the mirror.
Using another platform?
Something went wrong with the install?
Want to do things the old fashioned way?
That's OK. That's why there are manual installation instructions: