In many a 'How To' manual and elsewhere you are told to first write a synopsis. Secondly you are advised to start at the beginning. I do neither of these.
If I did there would be a blank page for a long time! First and foremost, I write, ideas for scenes at all stages of the book pouring out. When the words cease to flow, I rest (never hurts to grab a cappuccino and an almond croissant). On returning, I map out the order of the various scenes I have written and consider a synopsis to help tie it all together more succinctly. The spaces that remain, I also map out as they need to be filled. Research usually begins again here in earnest, as often whilst searching for titbits you are presented with wonderful ideas for prose as the result of the discovery of some implement or place. This jigsaw-piece approach works well for me, but I do not recommend or advocate any method. Each writer finds their own style and organisational techniques. Above all, I simply believe a writer should 'write'.