There's going to be many different approaches to creating material.
I think the 'best' approach is essentially problem solving. Identify an issue with an existing method, determine a possible solution, try it. If it works, refine it further.
When Aaron Fisher was creating the One Handed Pop Over, he did so because he wanted a one-handed version of an existing move. So he sat down and went step by step through the move to figure out how to turn it from needing two hands to being one handed. That's a fairly unusual case, though - usually the way a new method or presentation is created is due to the performer tweaking and honing the method or presentation over years and years of performing it.
You will also see folks who create something simply for the sake of creating something new. This is generally done with the distinct intention of selling that method or trick to the magic industry and in my opinion these offerings are usually under developed and not real world practical.
Personally I always develop material from the end backwards. Meaning I think up an experience I want to provide to an audience, then I script and block it, -then- I think about how I'll accomplish it. If I don't already have a method up to par, I learn or create it.