Follow the steps to cast Amazon Prime Video through Chromecast: 

1- Setup: Make sure your Chromecast-enabled device and iOS device are connected to the same Wi-Fi network.

2- Install Amazon Prime Video: Download and install the Amazon Prime Video app from the App Store.

3- Open Amazon Prime Video: Open the Amazon Prime Video app and log in to your Amazon account.

4- Start Playing: Choose the movie or TV show you want to watch and start playing it in the Amazon Prime Video app.

5- Casting: Look for the casting icon within the app. This icon is usually located in the top-right corner or in the playback controls. It looks like a rectangle with a Wi-Fi signal in the corner.

6- Choose Chromecast: Tap on the casting icon, and a list of available devices to cast to will appear. Select your Chromecast-enabled device from the list.

7- Start Casting: After selecting your Chromecast device, the video should start playing on your TV or display.