patxi:
From my little experience I cannot begin a story until the whole sequence, plot, characterisation and ending are firmly decided upon and fixed in my mind. What puzzles me is that I cannot write starting from page one and carrying on through to the end. I'm so eager to write down the good bits, it all comes out in no established order, no logical sequence so that as time passes I have a series of lengthy paragraphs which I polish up and stitch together, so to speak, in one patchwork quilt.
Does this make sense to anyone?
Of course it makes sense. Although I work in a different fashion, everyone is different, and should use whatever works. Like you, I don't start writing a story until I have it pretty much worked out in my head. Then I tend to write it straight through with very few revisions. With the logic having all been worked out ahead of time, I would never find myself writing patchwork and then stitching together. There are times that the plot strays somewhat from what I had planned out ahead of time, usually because my characters won't behave as I had intended them to, but even then, the writing (once I get to the actual writing) is all very linear, and won't change much when I return for final polishing. What this means is that I very seldom write anything too long, because it's too hard to have a very long story worked out mentally to that degree. But it makes it fairly easy to finish what I start - which is important for someone prone to writer's block.
But that's just me. As I said - everyone should use what works for him/her.