Pluck the middle part and shape the inside of your mouth like your saying “Oh”.

Once you get that, pluck it again and change your mouth like your saying “Eee”.

You can make “Boing” sounds be changing between “Oh” and “Eee” quickly.

Now try Breathing in and out through your mouth in rhythm to your strumming and it will change the sound a little.

Now experiment and have fun!!