Planning a campaign is a pain in the arse, though a pleasant one.
When you are on a major project, there always are two possible approaches: over Broadth or over Width.
YOu can either take your time to write one mission after the other, never moving on till everything is complete, or you write a mission, lay it out roughly, them move on to the next. While writing, you will get thousands of ideas, things which could blend in here or there, nice logical transitions, developments, reasons for the characters to do this and that...
Dont think in categories of "ships arrive, shooting starts", think in categories like "WHY are we doing this, what does our side want to achieve, how would the others react, what are the peoples personal agendas....."
My approach was the following ( on EACW )
Starting with a number of textfiles. One textfile for the campaign overview, listing missions, characters and so on, and for every mission an own textfile where you list the stuff that is supposed to happen IN DETAIL, notes, planned dialogue lines and so...