The process for unlocking a phone's bootloader varies depending on the manufacturer and model of the phone, but in general, the steps are as follows:
You must first enable Developer Options on your phone. To do this, go to Settings > About Phone, and tap the "Build Number" 7 times until you see a message saying "You are now a developer!".
Then, in Developer Options (which should now be visible in Settings), enable USB Debugging.
Download and install the appropriate phone drivers on your computer. These drivers are typically available on the manufacturer's website.
On your computer, download and install the Android SDK (Software Development Kit). This includes the tools required to unlock the bootloader.
Start the phone in bootloader mode. This is usually accomplished by turning off your phone and then pressing a specific combination of buttons (such as the Power and Volume Down buttons) to enter bootloader mode. For the specific combination of buttons to use for your phone, you may need to consult your phone's manual or search online.
Using a USB cable, connect your phone to your computer.
Navigate to the folder where the Android SDK is installed by opening a command prompt (on Windows) or terminal (on Mac or Linux).
To ensure that your phone is properly recognised by your computer, type "adb devices" into the command prompt or terminal.
Finally, type "fastboot oem unlock" to unlock the bootloader. This will erase all data on your phone, so make a backup of any important files first.
It is important to note that unlocking the bootloader may void your phone's warranty and make your phone more vulnerable to security threats. Proceed with caution and only unlock the bootloader if you're sure what you're doing and have a good reason to.