I found a nice guide to installing Retropie on Ubuntu 18.04.03 at a website called markontech and it works brilliantly.
sudo apt-get install -y git dialog unzip xmlstarlet
git clone --depth=1 https://github.com/RetroPie/RetroPie-Setup.git
Once the Retro-Pie install script is running, you will want to do a Basic Install and then navigate back to the menu and install the desired optional packages.
One thing I have learned: if you copy the retropie directory to a thumb drive (once it’s setup) each time you have a new setup, if you plug the thumb drive in, the computer will automatically copy the roms and bios files to the new install when emulation station is running. 🙂