Now that you've created a signboard or a slideshow, the next step is to get it on that display for your customers to see!
To cast to the display, make sure your Chromecast is connected to your display and both Chromecast and display are powered on. If you need additional help, please refer to the help section found here.
Make sure that the app is connected to the Chromecast. If both your iPhone and Chromecast are on the same wifi network, you should see the Cast icon at the top right of the app. If it's not there, it means you're not on the same wifi network.
Tap on the Cast icon to bring up a list of available Chromecasts. Tap on your Chromecast that is connected to the display:

For a signboard, tap on it to bring up the options. Tap on the Cast icon. For a slideshow, tap on the Cast icon:

You should see that it is now on the display!