Not difficult to make although you need a reasonable amount of thyme (so a healthy good size plant if you are to keep making it).

Add a small handful of thyme to 1 pint of warm water and leave it to steep overnight. Strain the thyme out and use the liquid to spray brassicas to keep the Cabbage White Butterflies away (if you have already got caterpillars the tea spray will bring them out of hiding so that you can remove them). Repeat as needed.