You need to download the Google Home app from Google play and install it on your Android phone. Plug the chrome cast dongle device into your HDMI port on your TV and make sure the device is also powered on (plugged in).
You then need to sync your TV to you phone by opening the Google home app and select your listed device . Input the unique cast name you make up. Save.
Then input the 4 digit code that is displayed alongside the new name you created previously. You should be connected at this point and see your phone screen on the TV. Other apps such as YouTube have their own app that you get from Google play. Download it, open it, select video to watch and then select the small Chromecast symbol on the YouTube app to sync it to your TV. Sometimes, the YouTube app will become it's own guide connected, so you need to close the YouTube app and also disconnected from Google home to reset it and display what you want to from your android phone.