Go out on a Sunday morning. Walk around your neighbourhood. Knock on doors. Offer to wash cars.
It’s incredibly simple, and over time you should be able to build up a round of cars that you wash once a week on people’s drives. You do a great job (never try and do anything less) and you save people having to take their cars to be washed (so you are convenient).
Say you charge £5 a car and you can wash three cars per hour. You could build up a round of, say, 20 cars, and so for a few hours on a Sunday you can pocket £100 every week.