I think KiraOnWhite said it before I could (and probably more eloquently at that) but I would add this:
From my own personal (little bit of) experience, regardless of what happens with book 1, if there's a book 2 that you've already imagined, you're going to write it someday (probably) so you as well get right in on it, at least to capture the flavour while it's freshest.
As for step-by-step instructions, that's all variable depending on the situation which could best be answered by an editor or perhaps a trusted friend who loves the genre you're writing in.
One possibility is that half the threads from book 1 would still be unanswered by the end of book 2, leaving you more options for book 3 and onwards. To decide which bits are like this, I'd say that if your editor or trusted friend asks you specifically about one bit, then you need to flesh it out sooner rather than later.
As the writer, chances are looking at your own work will give you a headachefull of loose threads which the reader might only notice half of.
So I think the best thing you could do if you did start writing book 2 while book 1 is still being edited is to decide which threads need to be answered soon. By deciding this, you'll notice some threads that you don't want to flesh out, so they can be cut willy-nilly during editing without giving you worry (if they need to be cut) and you'll also have a better idea of why certain bits have to stay in, which most reasonable editors will concede to if you explain why that loose end becomes a great thing in the next book. You may still have to rewrite the loose end to make it up to scratch (again, trust your editor) but you'll be able to keep it in there if you know it has to go in the next book or two.
Basically what I'm rambling at is be aware of every thread in there and what it needs to do. Read through the MS and make notes of what is unanswered, then simply put them in columns:
A) Ditch it.
B) Could be useful much later on (ditch it now could work, so long as you reintroduce it a book or two before you want to flesh it out)
C) The "Must have" pile - fight for them, but still listen to advice from editors/readers (I knew you would!)
Cliff