I spent a couple of months thinking about this - certainly, longer than I had intended. Admittedly, while I published a post on why writers should have babies, it is also true that babies eat your writing time. At best they do it with a goo and a gurgle. At worst - well, in case you're about to eat soon I'll spare you. So let's just say there can be days, sometime weeks when you can't get to a keyboard. This isn't always a bad thing.
Yes, in a perfect world I would write everyday at a set time, but when I'm thinking about a new idea sometimes not being able to write is a godsend. It's all too easy too kill an idea by exposing before it is ready. Never, never, tell a friend what you're working on until an idea is established both in your mind and on your hard disc. The innocent, but searching questions of friends, will kill many an idea on the vine.
When a new idea pops into my mind it has problems, often lots of problems. Mulling it over, making the odd note until I am desperate to get to the keyboard, if it's a good idea, gives it a life of its own. It matures in my brain and gets to the point where it's pushing to be born. And if you let it out at that moment there ain't no stopping it. (Ask any woman who's been in labour.)
I'm not suggesting for a moment that this formed idea with be perfect. It will need (again with the baby simile) to be nurtured, schooled and generally tended. Personally, I will often write an opening (which may or may not make it into the final version), then construct a plot outline and then day by day start winding out the text. As I'm writing ideas will often pop up in the unlikeliest of places which may mean a rethink or an edit of the outline. It's an organic process with the chicken and the egg constantly vie-ing for first place.
But this is how I start. How about you?