Simply earn $1,000,000 in the single player game. You don’t have to have it all at once. You can see your progress by going to the pause menu and selecting "Stats" > "Statistics", then scrolling down to "Total Money Earned".
1. Find a Titan, a big, boxy truck that says “Armored Truck” on it. These are pretty rare, though they may appear more often around banks and the High End Retail district. Steal it, drive it back to a crib or Rim Jobs and store it in the garage.
2. Go to the far northwest Rim Jobs, retrieve the truck, and drive up to the gas station pump. Get out and shoot at the pump to blow the truck up. Pick up the money ($4,000 - $12,000), go back inside Rim Jobs, repair your Titan, and repeat
2a. Alternatively, you can unlock the Platnium Rocket Launcher after completing all the Hitman activities. Then, retrieve the Titan at a crib and blow it up inside the garage. Just run inside to replenish your ammo when you run low.