I always hear this form of the request with a bit of bemusement. The simplest way to add $100K to your income is to rob a bank. You didn't constrain the problem to "legally" :-)
But setting that aside for a moment, why $100K? What does $100K get you? And why is money your goal? Do the thought experiment where "Poof!" the magic genie drops $100K on your bed. Sit there and look at it, now what?
You need money to pay the bills and survive, okay. But having it is rather fleeting, and when you die you cannot take it with you. Get past "money" and into "accomplishing something." When you have something you want to accomplish come back and we can opine on that :-)
$100k lands on my bed. Poof go my student loans and credit card debt. With enough left over that I could take a nice vacation and take a 6 month break from work if I live very frugally. Sounds like a worthy goal.
Edit: this is why claims of finishing returns with the ratio of earnings/happiness are unconvincing to me, or at least setting them as low as $70-80k (which some people do) seems wrong. Additional capital produces happiness because it provides security -- I can afford not just to service debt, but to eliminate it, making me free to take a lower paying job in the future. I can afford to buy a house outright instead of just a down payment. I can fund my own business ventures. I can provide for the people I love. I can bend the law and government to my personal interests. I don't see a real cap where more money stops providing more happiness and peace of mind, at least not before reaching sums that make you truly wealthy. Any middle class person in America is still incredibly subject to the vagaries of fortune, and the more money you have the more you alleviate your risk.
N.B., I personally can't afford any of these things.
FWIW I get/understand the goal directed version (exemplar: I'm looking to pay off credit debts (student loans, CC) and save for down payment on a house.)
The fastest way to make money "now" is to sell something you already own.
The second fastest is to create a service business which pays for your labor.
The third fastest is to sell things for someone else, where they let you keep a commission on everything you sell.
The fourth fastest is to take a professional job (Doctor, Lawyer, Engineer) at a large firm, and work your way up to a senior position.
The slowest is start a company, build a product, make it successful, and then sell the company.
The tricky bit is navigating the path in a way that you enjoy or are fulfilled by. My observation is that not doing so puts you in a less useful place at the end of your life. But I by no means have a statistically valid sampling of that.
An extra $100k in annual income could make the difference from maybe never retiring and having enough additional income to invest in a quality retirement where you and your family don't have to spend the final 10-30 years of your life compromising and living like misers.
But setting that aside for a moment, why $100K? What does $100K get you? And why is money your goal? Do the thought experiment where "Poof!" the magic genie drops $100K on your bed. Sit there and look at it, now what?
You need money to pay the bills and survive, okay. But having it is rather fleeting, and when you die you cannot take it with you. Get past "money" and into "accomplishing something." When you have something you want to accomplish come back and we can opine on that :-)