Kim Palmer has an excellent essay in the New York Times on her personal experience of creating a side business to supplement her day job
As a reporter working in a struggling industry, I always knew unexpected job loss was a possibility, but now, the stakes had never been higher. Being so financially vulnerable was unacceptable to me. And one day, as I was preparing to interview a businesswoman who made her living by selling cutting boards on Etsy.com, the handmade marketplace, I thought I had come upon a solution: I could have a side job making things to sell on the site.
By adding a second stream of income, I would no longer be entirely dependent on a single paycheck. As I explored the website to prepare questions for my interview, I discovered a section of calendars and planners. Some of them, like meal planning and budgeting sheets, looked like something I could create. What if I started an Etsy shop of my own, selling money planners based on my years of personal finance reporting? I could create workbooks based on different life goals, including having a baby and budgeting. What if I could become as successful as those Etsy sellers who crochet scarves and carve furniture?
She goes on to describe how she's slowly built up a 2nd income stream through trial and error and paying attention to what she's learning from her customers and their needs.
I work with a lot of people who decide to pursue entrepreneurship only after they've lost their jobs and are desperately looking for a way to bring in income while they look for full-time work. This is one of the hardest and most stressful times of your life to do this because so often you are freaking out about paying the bills. You may have more time to work on the business then, but you also put more pressure on yourself for it to be successful immediately.
The best time to start working on another income stream is when you have some security already. That way you can give yourself time to build up the business without the pressure of needing to replace your full-time income.
When I started my company 15 years ago, it took me a couple of years to finally bring in enough revenue to replace what I lost when I quit my job. Most businesses are not "overnight successes."
Do yourself a favor--start now on that side gig. That way you're ready to ramp things up if and when you need to.