Google Slides
Create a new Google Slides file and name it. Put something on the first slide or two just for testing. The whole slide show does not need to be complete for testing purposes. Click File -> Share -> Publish to the web Click Publish. DO NOT change any of the "Published content and settings" options, namely the "Restrict access" option as this will cause the signage screen to display a Google sign in page.
PlaySignage
You should be all set now. You can now click on the Preview button in the top right corner to view what it will look like.