There are dozens of ways to generate passive income. However, the option you select has to do with two metrics: time and money. Either you have a lot of time or a lot of money. Most people usually don't have both. But, if you have a lot of money, generating passive income almost instantly is easy. You can buy up some real estate and begin enjoying rental income. Or, you can invest in a dividend fund or some other investment vehicle that will begin generating a steady income for you.
If you have specialized knowledge in a certain topic, you can put together an online course to teach others. For example, if you have experience in real estate investing, you can create an online course “Real Estate Investing 101”. The benefit of an online course is that once you create the course material, you can sell it to as many people as you want.