Most of my ideas come from dreams--whether those be when I'm asleep, or just random daydreams. They tend to involve more plot than anything else. A bit of world-building too. So that's the stuff that I have in my head first. And that's about the time when I open up a word document and make a few notes based on what I've thought about, just so I don't forget it. Then I start wondering about characters, which sometimes I've already got an idea on them too, but I may or may not still need names (sometimes my dreams name the characters too....It's a little odd, but whatev).
The way Crashers worked was I was on my home for school, visiting for a weekend and the driver of the car and I started talking about how cool it would be to teleport home, and skip the 2 1/2 hour drive altogether. We both that it would cool, and that was the extent of the conversation, but my mind started playing with it, wondering about a world where that was possible--technologically--and then what would happen if it was destroyed? And then I combined that idea with another that I'd been working on, roughly, and BAM! There was Crashers. I still had to get names, but for the most part, I had the workings of plot, and a world to build off of.
As for the writing of it....I got a notebook and started writing down ideas for more plot stuff as they came to me. And I started working on the Prologue. And then Chapter 1. And I got stuck in the middle of that Chapter, so I focused the most on my notebook, getting further in plotting (up to Book 2 and some stuff in Book 3), and just waiting for the actual writer's block to wear off (I wanted to write it all in order) so I could write more.
For the most part, that's kind of how my writing process is. It's slightly different for each project that I work on, but those are the basics.