Hi Irene - here is a tutorial on making plastic pots look old with paint: https://www.dazzlewhilefrazzled.com/age-plastic-terra-cotta-flower-pots/
Here is a hometalk tutorial for aging pots https://www.hometalk.com/16461677/how-to-age-plastic-faux-terra-cotta-flower-pots
I thin out my paint of choice with some water, and then paint the paint on in thin coats. Once it's dry you can distress with 220 grit sandpaper.
I dry brush my pots with a white paint and then wipe off any extras