you'll have to have an AVR programmer set up. If you don't have that, click here to find out about different versions. You'll have to have some sort of an I2C connection already on the Raspberry Pi, click here to learn about this.
Some extra reading and resources:
On the theory of operation, calibration and precision check this application note from Atmel. On increasing resolution and oversampling look at this note. On a basic integer averaging technique and code optimization look at my post here.
Let's get started.
1. Download the usitwislave library.
2. Download the analogue digital converter C code from my github. Put them in the same directory.
3. I achieved reliable communication when the ATtiny worked at 8 MHz or above and brownout is set to 2.7V (see more on stability of I2C here). The fuses have to be set for this. Here are the avrdude options: