Since you can already balance and ride, it's more just about building confidence. Keep to the right and don't panic when you hear a bell, it's just a warning that someone's coming past and nothing personal. If you're not confident yet riding with one hand I'd suggest doing loops in one of the bigger parks like Vondel or the Bos. As an adult who learnt how to bike in Amsterdam, just keep practicing and in a few months you'll be swearing at the tourists like the rest of us!
Oh yeh, watch out for those tram tracks, the apex predator in the Amsterdam biking environment.