How did Supreme Court Justices get appointed during the American Colonial era?

There was no Supreme Court in Colonial America; the court system consisted of several judicial levels, such as justice of the peace, county courts, and supreme or superior courts, but they weren't part of an integrated system, like the federal judiciary is today.

There was also no Constitution at that time, and no formal procedures or guidelines to follow, so the courts primarily adopted British common law and applied penalties inconsistently, at the judge's whim.

In American Colonial times, judges and justices of the peace were usually appointed by the Governors of the individual Colonies, without the interference of other legislative bodies.

The Supreme Court of the United States was mandated by the Constitution, adopted in 1787, following the Revolutionary War that ended Colonialism in America. Congress established the Supreme Court and a limited federal judicial system in the Judiciary Act of 1789. The Court first convened on February 2, 1790.

George Washington nominated the first six (actually seven, but one declined) justices in September of 1789; the Senate approved their appointment a few days later. This process of selecting judges and justices was prescribed by the Constitution, and was unique to the United States.