The API test kit uses a small plastic container to hold a water sample from your tank.  After the sample is collected you use another solution to drop into that water.  Use 3 drops of solution to put inside your sample.
Once the water changes color, you can compare it to the api ph test kit color chart.
The color chart is on a piece of paper inside the kit box.  It is best to compare the water color to the chart in a well lit area.