These missions are better off done in relatively empty servers since other players will most likely hunt you down and try to ruin your money-making venture. Once you've filled up your warehouse, you can sell it all at once for a massive payday.
From there you can hire employees and via your CEO terminal/computer, you can choose from various types of goods, which you can then steal and store in your warehouse. The first thing you'll have to do is buy yourself a corporate office and start your own company. This method is for players who already have a decent amount of cars/possessions, but want to save up for that really expensive car, plane or yacht.
You can make a lot of money with this method, but you will need to have at least $1.5 million lying around in your bank account. The Further Adventures In Finance and Felony DLC gives GTA Online players the option to make money via some nefarious buying and selling ventures, as long as they have some money to start up their business with.
