Just complete one challenge. If you do this after you have done career all levels are farmiliar. I did mine in the black garden with enough time to spare. There are 2 type of challenges. Time and water. Time being clean the location or item in given time for gold or Water for efficiency.
Tips i can give is use the soap nozzle + soap on items you struggle with. This should give you a good edge on the challenge to come. There is another one for 5 gold stars