Statutory declarations in Singapore are governed by the Oaths and Declarations Act, and the form a valid declaration must take is fixed by the First Schedule to that Act. A declaration of marital status is not effective merely because you have written and signed it. The Act requires that you complete the document first, then sign it in the presence of a Commissioner for Oaths, who administers the affirmation and signs in turn. From that moment the declaration cannot be altered or corrected except before a Commissioner, so accuracy at the point of signing is not optional. You can read the operative wording and schedule on the official text of the Oaths and Declarations Act on Singapore Statutes Online at the Singapore Statutes Online entry for the Oaths and Declarations Act.
Who may take your declaration is equally prescribed. A Commissioner for Oaths in Singapore is typically a practising lawyer of at least 35 years of age with no fewer than 10 years of legal practice, appointed for the purpose. State Courts, the Supreme Court, ministries, and statutory boards usually have a Commissioner available, and most law firms offer the service. Where the declaration is made outside Singapore, section 12 of the Act controls: in a Commonwealth country other than Singapore it must be made before a notary public or a justice of the peace, and in a non-Commonwealth place before a Singapore consul, vice-consul, or another officer authorised to receive declarations. A marital-status declaration affirmed before the wrong person is worthless, so confirm the officer's authority before you attend.
The substantive law on marriage sits in the Women's Charter 1961, administered by the Registry of Marriages under the Ministry of Social and Family Development. The Charter is the reason a registry or consulate cares about your status: a person with a previous marriage must show it has been legally dissolved before remarrying, and divorcees who remarry are separately required to declare, by statutory declaration, whether they have existing maintenance orders and are current or in arrears. For Muslim marriages, the Administration of Muslim Law Act and the Registry of Muslim Marriages apply a different process, and a declaration intended for a marriage in Brunei, Indonesia, or Malaysia is usually framed to obtain a letter of permission from ROMM. The relevant requirements are set out across the Singapore family and divorce document templates.