Your phone/device should now be in fastboot mode.
Download the correct TWRP image file search here and copy the file into the same folder as your adb and fastboot binaries on your computer. Rename the just downloaded TWRP image to twrp.img.
On your computer type:
fastboot flash recovery twrp.img
Note many devices will replace your custom recovery automatically during first boot. To prevent this, use DuckDuckGo to find the proper key combo to enter recovery. After typing fastboot reboot on your computer, hold the key combo on your device/phone 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.