I agree with the earlier posts in here, so I just want to quote two advices H.P. Lovecraft once gave for technique/organisation:

I. Work out a rough plan with the scenes in chronological order
II. Work out a rough plan how the scenes shall appear in your story (flashbacks etc.)

Orientate by these two plans, but don't be a slave to them: feel free to change them anytime you think it's necessary.
Sometimes I'm worried about writing "non-fitting" passages, it really impedes me. So write without consideration of mistakes, bad grammar or even sense - you can always delete it.