Fastboot Install Method (No Root Required):
You will need the platform-tools from the Android SDK on your computer. Find the Android command line tools section on the page linked and install the SDK tools package. From the SDK Manager, download only the platform-tools to get adb and fastboot binaries.
Windows users will need proper drivers installed on their computer. You can try the
Naked ADB driversor the
Universal ADB drivers if you don't already have a working driver installed
On your device, go into Settings -> About and find the Build Number and tap on it 7 times to enable developer settings. Press back and go into Developer Options and enable USB debugging. From your computer, open a command prompt and type:
adb reboot bootloader
You should now be in fastboot mode.
Download the correct image file and copy the file into the same folder as your adb and fastboot binaries. Rename the image to twrp.img and type:
fastboot flash recovery twrp.img
fastboot reboot
Note many devices will replace your custom recovery automatically during first boot. To prevent this, use
Google to find the proper key combo to enter recovery. After typing fastboot reboot, hold the key combo and boot to TWRP. Once TWRP is booted, TWRP will patch the stock ROM to prevent the stock ROM from replacing TWRP. If you don't follow this step, you will have to repeat the install.