Here's how I work;
At first, I had a basic idea for a very short FF, something that I pictured to be like three chapters or so... The idea was simple, it was a futuristic plot of Naruto, where the final battle between Naruto and Tobi happened (This was in 2010 btw, when Naruto was still training with Killerbee in the manga). I thought of having Naruto at the Hokage Mansion, getting notified that Tobi had attacked Konoha, and the final battle play out... This was my original idea, before I wrote down anything at all. Then, when I wrote the first chapter, I quickly knew that this plot was too "hasten" to complete, and that I needed some kind of build up. So, I wrote the four first chapters as I went, perhaps even the six next ones too, and when I was at Chapter 10, I started writing down notes on what would happen next. Not only that, but I also had kicked off a Chronicle (within the same storyline), and was also planning to do another one (which I did). I figured if I was to make both these Chronicles match the storyline, without any plot-holes, I just had to start planning things, so I did. I never really had much pre-planning in my original work as I just wrote down simple notes on what I wanted to happen... It was, however, when I started to approach the end, that I felt like I had to make a choice - I would either display the final battle, or I would end when it was just about to start. I chose the latter... But when I did however, I started to get tons and tons of ideas for a sequel, too much to handle remembering, so, I began writing down notes... Lots of lots, of lots of notes. I basically had the entire sequel planned, long before I even reached the end of my original work. Heavily detailed notes on each chapters, even having some dialogues and scenes already written down as part of a chapter. Eventually, I figured that I needed more than a simple set of notes, so I started making several ones, including; "Notes 1 - Story, Notes 2 - Things to Remember, Notes 3 - Timeline, Notes 4 - Databook, Notes 5 - Changes and alterations for my future "re-release of my complete work," and of course, Note 6 - Links to all chapters, including Chronicles, In chronological reading order..."
I always knew the beginning and the end of my sequel, down to the smallest detail, but I only knew parts of what I wanted in the middle section, parts that I eventually formed from new ideas...
In conclusive to my post, my writing process is pretty much based on my notes these days, as I don't/can't write a new chapter without having my notes beside me at all time, double checking if I have all the points I've previously noted down. I do however have an idea of the basic plot, but I don't remember all the details that I've written down, so in some cases, I've written major parts of chapters that I won't be releasing for a year's time xD