There are three ways, and which one you can use depends entirely on the television.
Wireless, if the TV is a smart TV
Both devices must be on the same Wi-Fi network. Same network, not just both online.
Android to most smart TVs: swipe down for quick settings and look for Smart View, Screen Cast, Cast or Wireless projection. Tap it, wait for the TV to appear, select it. If the TV asks you to allow the connection, accept it on the TV with its remote.
iPhone to an Apple TV or an AirPlay television: open Control Centre and tap Screen Mirroring.
iPhone to a non-AirPlay TV does not work wirelessly without extra hardware. This catches people out. You need a cable or a device.
With a streaming stick
A Chromecast, a Google TV stick or a Roku turns any television with an HDMI socket into one you can cast to. This is usually the cheapest way to make an older TV work with a phone, and it is more reliable than built-in TV casting, which varies a lot in quality between brands.
With a Chromecast, apps like YouTube and Netflix show a cast icon inside the app. That is better than mirroring the whole screen: the TV plays the video directly, the picture is sharper, and you can use your phone for something else while it plays.
With a cable, which always works
When the wireless route fails, or when it matters that it does not stutter halfway through a presentation, use a cable.
- Android with USB-C: a USB-C to HDMI adapter or cable. Your phone must support video output over USB-C, which many budget phones do not. Check your model before buying.
- iPhone: a Lightning to HDMI adapter for older models, or USB-C to HDMI for iPhone 15 and newer.
Then set the TV to that HDMI input with its source button.
When the picture is bad
- Stuttering or freezing wirelessly: Wi-Fi congestion. Move closer to the router, or use a cable.
- Black bars around everything: normal. Your phone screen is a different shape from the TV.
- No sound: check the TV volume and input, and that the phone is not muted or connected to Bluetooth earbuds.
- A black screen while playing Netflix or DStv: this is deliberate. Those apps block screen mirroring to prevent recording. Use the app's own cast button, or the app on the TV itself.
For a presentation
Turn on Do not disturb first. Mirroring puts every notification that arrives on the wall in front of the room, including the ones you would not choose to share.