First off, you need a Micro SD card and a Micro SD card reader. Many modern computers comes with this built-in. The SD card must be 4GB or larger and preferably Class 10 or better.
Next you need to download the Screenly disk image. This process of writing this to an SD card is the same regardless if you’re trying to install Screenly OSE or Screenly Pro.
In the video below, you can follow along in the process from start to finish.
That’s it. Now you can just put the SD card into your Raspberry Pi and Screenly will boot.