When you see this sound transcribed phonetically, it is written as the first half of the symbol for A as in “cat” followed by the symbol for oo as in “book”. This might encourage us to produce quite a harsh sound. Something like “A-oo” - the sort of sound Henry Higgins hears from his poor student Eliza Doolittle whenever he frightens or upsets her. To make a less harsh and more open sound, make your starting position closer to AH as in “heart”. Don’t linger on it - move straight on to the short “oo” sound as quickly as you can: