I enjoy answering these questions as it really gets my imagination going. However to he honest there isn't a one approach fits all for it depends on several things like:

How much you need to make in order to survive
Your personality type and what you enjoy doing
Any skill sets you may have

Having said that, there are two ways of making money to keep you afloat whilst you work on your business. One idea is to get yourself a job.

While this advice may sound boring if not a step backwards into the employment world (especially if you are setting up a business to escape the 9-5 grind) bear with me. For I am not in any way suggesting you get a full-time job or even one that requires you to spend most of your working day in an office.

Jobs I am referring to can include being a delivery driver, after hours cleaner, part time salesmen. Even being a milkman in the early hours of the morning. These are the jobs which would provide you some steady income to live off whilst you work on your business.

Quick Note
Many years ago I briefly did some call center work and what sticks out in my mind about that job (and how boring it was) is the other people I worked with. For the vast majority of themselves were only working at the call center part time in order to keep them supported whilst they worked on their main enterprise.
During that time, I came across a number of entrepreneurs, authors, musicians, etc who were only working at the call center in order to keep themselves afloat whilst pursuing their dreams.

The other approach is to set up a small cash flow business. Now if you are devoting all your efforts to get one business off the ground, the last thing you probably want is to start another business.

Yet with this second business, I am referring to a quick cash flow business; one which is easy to get into and just as easy to shut down when your main venture takes off. Examples of these businesses include flipping items on eBay or freelancer on freelance marketplaces like Upwork, etc. You could also consider signing up to become a Uber driver or deliver parcels for Amazon using Amazon Flex.

The beauty of these cash flow businesses is that it doesn't take too long to get off the ground. And can also be done so in your own time.

Whichever option you go for, I wish you luck and if you ever want more advice and help on getting your business of the ground, do book a consultation with me.