Plug in to HDMI port on TV, give it power from the plug or via a usb socket on TV.
Now install the Chromecast app on your android or pc.
The app will search for active device and ask you to follow the onscreen task to connect the Chromecast to wifi.
Once thats done, you just start up your vidoe player or similar Chromecast app and then tap the Chromecast icon to send output to the TV.