If you know anything well, a place, how to fix something, how to make something, how to do something, you can write a guide for it. You can sell your guide as an e-book, offer it as a download for a fee on your site or reach out to bloggers with similar content and ask if they will offer it as a paid download on their website (for a price of course).

Another way to generate passive income is to invest and be a silent partner in a business. This is very risky, but with risk comes the potential for high returns. For example, several years ago both Lyft and Uber were looking for private investors to invest in their companies. Today, they are worth billions - but you as an investor would only reap that benefit if they go public via an IPO, or get acquired. So, it's risky.
Lending Club went public in 2014 and is now worth about $1.7B. They advertise P2P lending returns of over 7% for well-diversified portfolios of over 100 notes. I’ve personally been able to achieve a 7.4% annual return over the past two years in a completely passive way by investing in A and AA notes. Others have achieved a 10% annual return through relatively minimum effort.

YouTube brought in nearly $4 billion in estimated ad revenue in 2018. So you can earn a potentially large income by uploading videos to the platform and then watching those ad dollars roll in. You may need or want to upload videos regularly, which isn’t especially passive. But you don’t need to provide a specific product or service directly to customers.
It is very important to understand that contacting a “professional” to learn how to do this only results in them trying to sell me crap properties (whether high end or low end). I’ve tried contacting realtors out of state, and they attempt to sell me crap or someone else’s problem. No one has a vested interest in actually helping someone or teaching them about how to get an out of state rental. very frustrating. I could go out tomorrow and buy a rental in my city, but that is the last place I want to own one. Anyone? Are there an real people on here?
To save time and effort, a person can group two or more of their passive activities into one larger activity, provided they form an "appropriate economic unit." When a taxpayer does this, instead of having to provide material participation in multiple activities, they only have to provide it for the activity as a whole. In addition, if a person includes multiple activities into one group and has to dispose of one of those activities, they’ve only done away with part of a larger activity as opposed to all of a smaller one. 
Many people talk about passive income and create the impression that you never have to do anything to keep that income going. The truth is that you will normally have to keep your eye on things if you want it to run smoothly. For example Richard Branson doesn’t run any of the 400+ companies he started but he goes over the numbers each day to make sure they’re performing well and calls the CEO if there are any problems.
Though it can take a while to build up enough cash to put a 20% down payment on an investment property (the typical lender minimum), they can snowball fairly quickly. The key here is to correctly project income and expenses in order to calculate cash flow (the free cash you can put in your pocket after all associated property expenses have been paid). However you have to be sure to include the cost of a property manager in your calculations unless you want to manage the property yourself. Even with a property manager, you may be required to make large repair decisions every now and then – so while this is not a 100% passive activity, you are not directly trading your time for money like traditional employment.
I’m a 45 year old business owner who also has focussed on diversifying my income streams. I have a short term vacation rental in Florida that I bought for $390k in 2012 and net rental income for the last three years has been growing steadily. 2015 I am at $70k gross right now but should end up at $80-85k with net around $45k plus we use the place about 35 nights a year.
I live in NYC where I never thought buying rental property would be possible, but am looking into buying rental property in the Midwest where it cash flows and have someone manage it for me (turnkey real estate investing I guess some would call it). I agree with what Mike said about leverage and tax advantages, but I’m still a newbie to real estate investing so I can’t so how it will go. I have a very small amount in P2P…I’m at around 6.3% It’s okay but I don’t know how liquid it is and it still is relatively new…I’d prefer investing in the stock market.
As explained in the Investopedia video above, residual income may play an important part in successful loan applications. In fact, some of the safest loans on the market come with this approach to debtors’ income. One such type of loan are VA mortgages. These highly affordable loans frequently require no down payment (in the case of about 80% of such products on the market right now), yet they’ve also seen one of the historically lowest rates of foreclosure over the past 10 years. Some experts claim this is because of their residual income standard, which aims to make sure that veterans can afford monthly payments on their home loans. The amount of residual income required, in order to be eligible for a mortgage of at least $80,000 varies based on where you live, as detailed in the chart below:
Residual income is the best model for money generation. Once you master and build up one avenue, you can devote your time and money into another avenue. Eventually you start reaping the benefits of multiple residual income avenues. Enabling you to have complete financial and time freedom. I recommend to all people to build these types of asset models as they can greatly improve their life.