Because you have a plan

You’ve planned it down to the last detail: when you’ll quit, what you’ll do next, how much time you’ll need to get that next thing going, how much money you’ll need to tide you over until then, how much income you’ll need for it to be sustainable.

Or you’ve started making a plan.

Or you’re thinking about making a plan.

Whatever stage you’re at in your planning, you know what you need to do next.