Before you start, be aware that these instructions were tested on a Pixel 3 running Android 12, but the process for most current Android phones should be pretty much the same. Needless to say, first make sure that all of your data is transferred to your new phone, backed up, or both. Once your old phone has reset, there’s no going back.

    Go to Settings
    Select System > Reset options
    You will be presented with four choices. Reset Wi-Fi, mobile & Bluetooth is sometimes useful if you’re having trouble with connectivity. Reset app preferences will restore all of your app and notification defaults, among other things. Erase downloaded SIMs will help you delete any digital SIMs you no longer need. However, none of these will delete the data on your phone. That’s the job of the last choice, Erase all data (factory reset), and that’s the one you’re going to select.
    The next screen will tell you what is going to be erased, and which accounts you’re signed into. Select Erase all data.
    If you have a PIN or some other security setup, you’ll be asked to enter it
    You’ll get another screen asking if you’re sure this is what you want to do. Are you sure? Then hit the Erase all data button.

That’s it! The process usually takes only two or three minutes. Your phone will reboot and begin with the usual opening questions, such as language, account, network, etc. Once you see those, you’ll know that it’s worked.

There are a few things to be aware of. First, if you’ve got a Samsung phone, you may be asked for your Samsung account password before you can reset (provided you’ve created one and added it to the phone).

If you’ve got an Android phone that is using an earlier version of the OS (say, before version 8), the reset may, after it reboots, ask for your login. This could be awkward if you’ve already sent the phone to someone else. As a result, if you’ve got an older phone, it’s a good idea to turn off your screen lock (by going to Security > Screen lock and selecting None) and deleting your Google account (which you’ll find at Settings > Account > Google) before you start the reset. (In fact, that may be a good idea in any case.)

There is evidence that some data may remain on a phone even after it’s been reset. To be extra careful, you can encrypt the phone before resetting it:

    Go to Settings > Security > Encryption & credentials and select Encrypt phone
    Go back to your settings and start the reset process

Of course, if you want to be really sure, and you don’t intend to give the phone to anyone, you could always grab a hammer.