Please note, that the installation depends on the version of your Arduino IDE.
After the installation, you should have several ATtiny variants available which are ATtiny44, 45, 84, 85 with speeds of 1, 8 and 20MHz for a total of 12 new entries in the Tools > Board
menu of the Arduino IDE.
cd /tmp
wget https://github.com/damellis/attiny/archive/ide-1.0.x.zip
unzip ide-1.0.x.zip
sudo cp -r attiny-ide-1.0.x/attiny /usr/share/arduino/hardware/attiny
wget https://github.com/damellis/attiny/archive/master.zip
unzip master.zip
sudo cp -r attiny-master /usr/share/arduino/hardware/attiny