Normally I'm a pantser on it. I put down the initiating event and head into the valley:
Writing a novel is as if you are going off on a journey across a valley. The valley is full of mist, but you can see the top of a tree here and the top of another tree over there. And with any luck you can see the other side of the valley. But you cannot see down into the mist. Nevertheless, you head for the first tree.
however, there are motifs that look like they will happen again and again. Motifs that will pose a danger and challenge to our young heroes. And not irregularly. Indeed, one will happen more or less once a year -- someone thinks one of our young heroes is magically advantageous and exploitable -- and the other will happen every year like clockwork, because the kingdom calls for young wizards to vie against each other, the theory being that will encourage them to study hard.
Which means it will really help to know in advance what they are, because sequence is important. Challenges have to increase, to challenge them more each time. And if each one is integral to the plot, I need to know before I marry it to the events around it, because it's hard to pull them loose.
Time to brainstorm, it seems.