Hello,
Using Etcher, use this image on the SD card:
http://downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2019-04-09/2019-04-08-raspbian-stretch-lite.zip
(if you need to control it via SSH: https://github.com/Screenly/screenly-ose/wiki#enabling-ssh)
Then, when you are on logged on to the raspberry pi, with user pi and password raspberry, and the RPi has internet connectivity by configuring it via typing in console sudo raspi-config and setting up the timezone and all of that, type this to install screenly-ose master image:
bash <(curl -sL https://www.screenly.io/install-ose.sh)
press enter and follow the instructions.