Since Plex offers offical packages for raspberry systems, installing plex got really easy. The official installation documentation can be found here but misses a few steps when setting up on a freshly setup raspi:
curl https://downloads.plex.tv/plex-keys/PlexSign.key | sudo apt-key add - sudo apt install apt-transport-https echo deb https://downloads.plex.tv/repo/deb public main | sudo tee /etc/apt/sources.list.d/plexmediaserver.list sudo apt-get update sudo apt-get install plexmediaserver