This guide walks through how to install RetroPie on an existing Raspbian install.
Download and run the install script.
git clone --depth=1 https://github.com/RetroPie/RetroPie-Setup.git
cd RetroPie-Setup
sudo ./retropie_setup.sh
Optional: Enable TFT display.
curl https://raw.githubusercontent.com/adafruit/Raspberry-Pi-Installer-Scripts/master/adafruit-pitft.sh >pitft.sh
sudo bash pitft.sh
https://github.com/RetroPie/RetroPie-Setup/wiki/Manual-Installation