First, (if you haven't already) you will need to create your presentation on Google Slides.

Once the presentation has been created, you will need to click on "Publish to the Web" from the drop-down menu of the File item.

The selecting the "Embed" section, you will need to copy the link as show in the picture.

Go into WallSign and then create a new Web Pages resources pasting the link into the url field. 

As shown in the picture above, there are some options that can be customized as the "Auto-advance" time and the autostart of the presentation.
The Slide size setting is ignored beacuse the presentation will fill the zone on screen where it will be displayed.

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