This part's easy—just finish at the top of your class at one of the best law schools in the country, amass a stellar legal résumé, and be in the right place at the right time without any skeletons in your closet. The only technical qualifications needed to be eligible for the Supreme Court are 12 years as a practicing lawyer or judge and five years in good standing with the bar; however, only the best and brightest legal minds are considered for Supreme Court appointments, and then there are the factors beyond anybody's control.
For example, is the President a Republican or a Democrat? Since the President appoints Supreme Court Justices, he's only going to choose somebody who agrees with his basic judicial philosophy. This is a ridiculous tightrope act—cling to the middle too hard and you won’t please either side, but push too far right or left and the necessary votes for confirmation will disappear. Not only that, but because of the desire to maintain a diverse court (an entirely different animal than a divorce court), a specific ethnicity or gender may be required for a given seat. Ultimately, the necessary qualifications for the Supreme Court are a mix of great achievement and more-or-less winning the lotto.