I get the idea and let it grow in my mind. I start mentally writing out scenes and developing characters, their personalities, quirks and back stories. Then I start the actual writing process but it's always slow to start with, I might only write a couple of hundred words a day at first - if I'm lucky. Mostly because I'm still mentally doing the work. Once all the imagination and mental work is really done then my writing tends to speed up to where I'm writing thousands of words a day.
I don't know if this is true of all authors obviously but for me so much of the work comes true in my head first and on paper later. I love sitting quietly with a cuppa, thinking out the overall story arc, planning major plot points, seeing how I imagine my characters will grow and behave.
When I'm actually writing though so much of that tends to change, the characters, the story takes over so that I'm usually surprised by the ending myself. I often get taken unawares by the direction a particular character will move in or how they will react to the different stresses and pressures that I add into the story. That's also a truly fun part of the creative process - watching how it changes from what you thought it would be to what it eventually becomes.
I'll probably bore you all from time to time as I'm working on the new fantasy series, I hope you'll bear with me as I share the journey of writing with you.