For me it's a two-step process to write the actual notes. The first step happens in class, copying down what I think I need to understand the concept being taught. Take cues from the teacher, if they write something on the board or it appears in their slides it's probably worth noting.

The second step is much more important. As soon as possible after class, hopefully armed with a better understanding of all the material, I write my actual notes. I write down everything I think I'll need to study to remember what I need to remember. Then I make an Anki deck from the notes some time in the next day or two and by that point I'll have the material pretty much committed to memory. I review the Anki deck a couple times over the next week to really lock it all in.

That's the process that's getting me through helicopter fight training now, currently working on my CFI.