Detailed instruction is available on the Instructablses
http://www.instructables.com/id/Digital-3D-Printer-Filament-Counter-Use-PS2-Mouse/

1. Collect the parts

2. Print the sensor body and control box

3. Take the mouse apart, get the circuit board and one encoder wheel

4. Assemble the parts

5. Flash the Arduino UNO with the sketch

6. Mount the sensor in the filament path, you're good to go!