Simple answer: save six to twelve months of living expenses from your day job so you can quit. Then you'll have plenty of time to make traction happen with a side project; and turn it into something real. Cut expenses as much as possible so you can build up your savings to the required level.
More complex answer. Compartmentalize your day job into the hours you put in there. Look at all other time as side project or freelance time. Get rid of all debt, if you have any. Getting out of debt is one of those liberating things that will make quitting your day job much easier. This second option is more time-consuming. You might find yourself taking on more responsibilities that you can't easily escape. Try to find a way to quit your day job sooner, rather than later; otherwise you may never quit.