Open the Arduino software and paste the Arduino code (the text in the box near the bottom of the post) into a new sketch. Click Sketch > Verify / Compile to make sure the code is properly in there.

The Arduino Sketch (code) uses the external interrupt (int 0) on Arduino’s digital pin 2 (D2). This is used to read the output pulses coming from the water flow sensor. When Arduino detects the pulse, ...