1/2cupLemi Shinethe original instructions call for citric acid which is cheapest when purchased online. Otherwise it is available in the canning aisle during canning season at a higher price. It didn't work very well for me. On their FAQ page they mention that those with hard water problems may like to try Lemi Shine and I found it works perfectly. It is available at Walmart, which I found more convenient than trying to hunt down citric acid. You can play with increasing or decreasing the amount depending on how hard your water is. 1/2 cup worked for me.
Mix everything together in a lidded container and shake.