First make sure that your Smart TV is capable of Screen Mirroring (ShareCast). If it is, your TV should have a ShareCast or Screen Mirroring or whatever it was called that year (Samsung seems to like to change the name of this feature every year). Next, set your phone's screen mirroring switch to ON and that's it. As long as you're in the same WiFi (you can also connect through WiFi direct without a router needed if your TV supports it) and the router doesn't block the video stream, you should be able to mirror the screen.
And if your TV is one of the newer ones and supports it, you can also mirror your TV in your phone using the Smart View app
Mine does, and now I don't miss a thing about any show or movie when I have to go to the bathroom or cooking or whatever.