I've been thinking about this for a short while last night because I was going to be starting one for Xenoblade 2. A novelization is basically a form of fanfiction where you write a novel adaptation of something, which I often see with Pokémon games. Of course, how much planning your novelization will often depend on your subject and how many extra things you intend to handle. The first thing you need to do is thoroughly research the subject of your novelization. Pokémon games are pretty straight forward: just play the game, and research moves and stats. But what about other games? Let's look at Fire Emblem Awakening. You have a ton of main story chapters and characters, even a lot of side chapters. Let's say we want to bring in all of the future children. That alone takes careful planning and consideration. You can't just be in the middle of the Walhart chapters and suddenly go recruit Nah in a novelization especially all willy nilly, because her side chapter is all the way on the other side of the map. You have to have a good reason for the characters to go out of their way. You have to think about when they're going to be in that area as well, or else space out major events some way. Let's look at Xenoblade 2 as well. You can't just have the characters go "Okay, let's leave the The World Tree and do something else." for a Blade Quest. Realistically, it doesn't work that way, and how would they make it back to say... Tantal for part of KOS-MOS's Blade Quest anyway? Some things just don't work. Okay, so Zenobia, Poppi, and Herald could possibly just fly everyone there, so it's not really the best example of something that's impossible. So short version, any novelization needs 3 things: research, attention to detail, and careful planning. Basically the same things as regular stories. It seems easy at first, but all that's really done for you is the main plot. The rest can be a pretty intensive undertaking. The rest is all up to you, including whatever alterations you may decided to make.