Put your phone into Bootloader Mode and fastboot flash the TWRP recovery file you downloaded. (extract the img file from the rar file first) I'm not gonna go over how to use Fastboot here as there are many guides on many different websites on how to use it. Then do a fastboot reboot. When your phone reboots shut it back down and then hold in volume up and power until it turns on and keep holding in volume up until the phone boots into TWRP recovery.