On top of start small I'd add: just finish it first. Maybe consider giving yourself a deadline. I got my first tabletop RPG supplement out last year because I signed up for (and got accepted into, horror of horrors) a table at a games/comics/arts festival. I had to go from idea to printed product in three months and worked my ass off in between my day job and taking care of my family to pull it off. But I did it and I am glad: finishing something has opened up my eyes to what's possible.
That being said I also took some time off of work last year to explore making games. There's a flourishing, inclusive indie developer community in my city and that was a real boon. However I'm still not sure how/if any of them make a living from their games.
Is it actually possible to aim your sights on a career in indie game development with all the responsibilities of a mortgage and family?
I want to do more games stuff but I foresee it being a hobby more than anything.