1) Arduino IDE Environment
Click on the "attiny-master.zip" link above and download it. Save it to somewhere where it is easy to find. We're going to need it later.
Start your Arduino IDE Environment and click on File followed by Preferences.
Open that exact same directory of your "Sketchbook Location" on your computer file explorer. Create a new folder and name it "hardware". (Note: You don't have to create a new folder if a folder named hardware has already existed)
Extract the content of the zip file (attiny-master.zip) into the hardware folder. Note: A folder named "attiny" and NOT "attiny-master" should be in the hardware folder. You should have a directory of something like this:
Close any Arduino IDE Software if you have it running. Restart the Arduino IDE. Click on Tools followed by Board. You should have the all the AtTiny option there.
Basically, that's about it. Your Arduino Software is now ready to program AtTiny chips.