On your Mac or PC, make sure you are running the latest version of Chrome (by going to chrome://settings/help) then start the video player from your My Stash page by clicking WATCH NOW. 

On Android or iOS access the Webcasts from the My Stash section of the LivePhish app. 

Once the video is playing click the casting button in the bottom right corner of the video player, then choose your Chromecast device from the pop-up menu.

Be sure to start casting within 30 seconds of starting the video from your Stash otherwise you may see an authentication error.