Pretty simple, but a little laborsome. Start with distilled water. Find a calculator online and measure out a precise dose of NO3
(for example) in a measured quantity of water. So basically you know you have created a 10ppm solution say. Run that through your test kit and compare it to your color chart for accuracy. Then make a 5ppm, 20ppm etc ... keep these samples and compare against your tank test for a much more accurate reading.
Kind of a pain though.