The Roku Express 4K Plus is the best streaming device overall right now. We prefer Roku's content-agnostic approach, ease of use, large app library and excellent search functionality over those from other companies. The Express 4K Plus is also usually the cheapest Roku that offers 4K HDR streaming.
For example, if you're using an Android device, you must go into Settings > Display > Cast menu and enable it from there. On iOS devices, this can usually be found under Control Center > Airplay (or Screen Mirroring). With your phone enabled for screen mirroring, it's time to turn on your smart TV's feature.