Rooting on Lollipop HTC M8 topic

Hi,

I recently updated my phone to Lollipop. I am looking to root it. This will be the first time that I will be rooting my phone. I have unlocked the bootloader but when it comes to updating my phone to install TWRP I get the message "Download complete. We will now attempt to reboot into TWRP and install SuperSU. If this app just closed, try rebooting into TWRP manually". I click continue but nothing happens. I am also unsure how to manually update this? Please help I'm a noob to this stuff.

Thanks