I've read several interviews with authors, and many of them work odd hours (early morning, late night), when everyone is in bed.
Turn off the internet, you know we'll only distract you... (i.e, don't have the browser up)
I start with an idea. Usually...'ok, that's a cliche`, but what if this happened instead?...
Most of this tends to happen in the shower, just waking up, going to sleep, stuck in traffic, usual places...
I write (badly.., and what I tend to do is just let the story flow through my mind, deciding on various dialog, details, plot twists, etc. I'll sit down and type the 'bones' (what I call it... of it, hanging disjointed scenes together. Then I'll reread, and rewrite, filling out descriptions, more dialog, etc.
Then I go through it, looking for spelling mistakes (thank you, Word..., grammar, continuity, etc. Then convert it to a suitable format and post.