Some proven ways that I did are.
Rental Income.
Market your online skills on Fiverr.
Become a freelance writer.
Also suggest me, some more sources if you also work on different sources of earning.
if you are a good writer, then you can start your own blog, where you can write and share your experience with other. After that you can monetize your blog and earn from them.
Other approach is that, you can shot your vlogs and can post on youtube, this is also a very easy and good way for side by side earning
Set up a blog or website about a topic that you are passionate, interest or have experience of.
For example, you could recommend online services to small businesses that want to sell online, so you can promote to them Domains, web hosting plans, email services, website templates and in this way earn several commissions from one client.