Start a boutique consultancy with a buddy. Hire up until you get to ~8 employees. Continue for 10 years. Congrats, you're a millionaire.
Everyone knows the management consulting / investment banking career path, right? Congrats, you're a millionaire.
Go to med school. Specialize in whatever the high-demand fields are, like e.g. anesthesiology. Execute competently for 10 years. Congrats, you're a millionaire.