It really varies based on how much "extra" time I have.
Since this is asking about creating an adventure, I'll just stick to what I've done when I've had the time
I'll usually start with the vast, over-reaching plotline that I want to convey. To this end I'll generally create the game world from scratch, getting at least a little bit for all the major sections down somewhere (paper, electronic, whatever is at hand) as well as at least a sketched map.
From there I'll decide where and how I want to get the PCs into things. Based on that and from some general background info from what the players want to play, I'll decide where they'll be starting. Sometimes it'll be a "you all know each other" start and other times it'll be a whole session just getting them all together and at least loosely becoming a party. So once I decide on that I'll more fully flesh out the locale for the first session.
Once I have a really good idea of where they'll be, I'll start going through what they'll be doing. I really can't say anything more specific on how I make that decision other than I just plant the seed in the back of my mind and let whatever extra cycles of my brain want to work with it go...
Once I have the plot for getting them "hooked" so to speak, I'll flesh that out, going to the lengths of drawing location maps, creating and completely fleshing out NPCs, populating monsters (if there are any at the beginning) and even planting the early seeds of things to come. This is the most important part, for me. I have my grand, over-reaching plot in mind, as I said, and the first adventure will have some subtle clues of what that might be.
As time goes on the story with the PCs will have some general sense of direction, but they're pretty much free to go as they want. Yeah, there are some plot points I'll want them to hit, so that they can start to see more and more of the big picture, but for the most part I'll wing most things between the start and the first milestone (I've even been known to pull entire castle maps out of thin air, as the characters are exploring it

)
After that it's just me letting them weave and wander, but bringing them in line with the grand scheme every once in a while only to let them wander off again. Sometimes I'll flesh out something that I want to throw their way, but it's usually more of a nebulous idea with some fleshed-out personality as part of it.
Don't know if that really answers your question, or even gives you an idea of things, but that's how I've done most of the stuff when I've had the time
Recently it's just been me working them from one published module to another, making little tweaks here and there to make them seem more ... personal for each person involved (or at least trying to... )