If you go to that site, the instructions seem somewhat complicated. And they do not state that it is desirable to apply a gesso product on the clay pot prior to painting on the pot. It is advisable to paint on the clear product so that the paint won't soak into the clay.
This site gives excellent step-by-step instructions for preparation of the pot prior to painting. That same reference will further explain the importance of preparing the clay surface for paint.
The picture on the right upper corner is one from the Michaels website. Near right photo shows a flower pot which I painted two years ago (using a clay sealer undercoating), which has held up fairly well. I used oil paints on this flower pot, but acrylics also might be applied for a quicker drying time. (The painting on the flower pot to the right was taken from an original Linda Le Kniff drawing.)
If you were to paint on the newer heavy duty plastic pots, prep time would be quickly shortened.
Remember to spray a clear acrylic coating on the finished pot for a bright, clear finish.