1) First of all, download the chromecast app on your mobile/tablet.
2) Open the app, select the 'devices' tab at the top of the screen.
3) Then, available list of chromecast devices will be on the screen.
4) Now 'Chromecast5108' something like this will appear on your HDTV's screen.
5) Now tap on 'set up' on your mobile's/tablet's screen with the same name on TV's screen 'Chromecast5108'.
6) Now verify 4-digit code and make sure that they are same on TV and mobile.
7) Finally, name your Chromecast and wait until the process is done. It may take few minutes.
(Also don't forget to add your WiFi's password when it asks)
Hope this helps!