I think the toughest part ends up being the time between when you leave your job and when you raise your first funding round. That could be a 1-2 month gap, or it could be a year. Best advice there is that you can probably do some of the initial startup work while working somewhere (e.g. customer interviews, spec'ing out a product, recruiting a cofounder, maybe even launching something), so that when you leave your job you can do a fundraise soon and have decent confidence in the company you want to build.