When there's an opening in the Supreme Court for a new judge, it is up to the president of the United States to nominate a candidate.
First, the president makes a nomination because the Constitution doesn't specify any qualifications, like educational background or place of birth.
Next, the nomination needs to be approved by the Senate. Traditionally, the Senate turns the nomination over to hearings by the Senate Judiciary Committee, where the candidate is thoroughly scrutinized.
The Judiciary Committee then votes to send the nomination to the full Senate with apositive or negative recommendation, or no recommendation at all.
With the Senate's consent, the final step is for the president to issue a written appointment. Then the nominee takes the constitutional and judicial oaths and has the job for life.