Often I get a new idea when I'm working on something else and have to fight not to be distracted. I sometimes jot down the idea and put it to one side until it's that book's turn to be written. I also need a bit of time to mull the idea over to check it definitely has enough potential to be a 100k word novel because not all of my ideas do.
I then flesh out the characters - I write bios for them and seek out actors or models who resemble them. This is usually when I start my Pinterest board. It's a handy place to drop in pins that help me to build up pictures in my mind of the characters and also the location. This is the one time I get to look at animals on the internet and it's not procrastination! There's always an animal in my stories and Pinterest helps me to pin down (do you see what I did there?) what they look like. At the same time I concentrate on what my characters are each trying to achieve or what they think their goal is - often the two things are different.
I then look at the plot. Often the initial idea and characters pop up together but it takes some work to really weave a robust plot. I jot down key scenes, plot points and cliffhangers and make sure I know any secrets that my characters are keeping so I can work out who else knows them and when they will be revealed.
Next I map out the whole story with post-it notes. I use different colours and sizes on a large piece of brown paper. These sticky notes get added to and moved around as the story builds.
When I get to the point that I'm itching to write the story I know I'm ready to start!
Here's the Pinterest board I made for The Promise of Summer.