Simplify the following text for a Grade 12 native English-speaking person.
Original: Then , on 3 December 1959 , the National Flag was unveiled at the installation of the new Head of State , the Yang di-Pertuan Negara , when Singapore was granted self-rule ( but not complete independence ) by the British Government .
Simplified: Singapore became self-governing within the British Empire on 3 June 1959 . Six months later , upon the installation of the new Yang di-Pertuan Negara ( head of state ) on 3 December 1959 , the national flag was officially adopted , along with the state coat of arms and the national anthem Majulah Singapura ( `` Onward Singapore '' ) .