Okay, you're probably looking at that title thinking, "What in the world are you talking about!?" Well, allow me to explain. When writing a series of books, especially one that'll be long, it's always wise to never write yourself into a corner. The easiest way I've found to do this is to write books in a series in such a way that they all splice together perfectly. To make that happen I've learned that, sometimes you have to completely write the next book in the series before you finalize your current one. Why? Well, 1) it creates less of a need to do any major rewriting. Especially if you see your story going in an unplanned direction. (believe me, once the story takes over, it writes itself pretty much. You're just along for the ride.)
2) You can better fix plot holes and story issues without doing major overhauls once a story is fully edited and polished. There's nothing worse than to get a story all nice and polished only to realize you have to gut and redo a section because it either paints you into a corner in the next book, or perhaps something that comes up in the next book needs to be foreshadowed or added to the previous book, etc, etc. Take my Offworld Chronicles series for example. Book 1 was fully writen initially, but then set aside while book 2 was completed. Once two was fully written and I was satisfied with how it was laid out and read, I finalized book 1, then moved on to book 3. Once that's fully written, I'll polish up book 2 and ship it, and start on 4 before I polish and finish up 3. And so on, and so on. Then, when I get to the one that will be the last book in the series, I will write that, polish the prior book, and then polish the last book.
Sometimes too, if you're adventurous enough, and the series isn't too long (What's considered too long? Ask the guy who wrote "Wheel of Time". That's just too ridiculously long.), you can write the entire series and complete all the books at once, clean and polish them all at once, and then when the entire series is ready front to back, release it. Typically that only works if the series is small, usually up to 5 books long. For longer ones, the method I use right now is best. It also works well if you don't know how long the series will be. As in the case of OWC, I have no idea how long it'll be. The novel it's based off of prior to my rewriting and expanding it can give you some idea of how it'll end, but not what's in the middle since I did a big time skip in the book at one point due to more or less growing bored with the story at one point. So I have no idea how far I'll be going on this before I finally get the team to the end.
Presently the future plot for the story has them taking one more winter stopover, and then finishing up their journey at the end of the 3rd year. That's the current "plan". But as anyone in the writing world knows, no initial plan ever survives the rough draft. ;) So we'll see. Anyhow, I just wanted to share this little tidbit with all of you who are aspiring writers. But the biggest I can give you is write, write, write and write some more. The more you write, the better you get. So write as much as you can because, if you love it, you'll never quit. If you don't, you'll burn out and walk away. But if you do love writing, you'll never quit. You'll only get better. :)