Computer programmers learn early in their careers the value of the divide and conquer approach; not to try and tackle a project in its entirety, but rather to break the work down into modules, and sub modules which are more manageable.
I’ve found technical writing and documentation in general to be no different. My approach in both programming and writing has always been to figure out what you want to write (or program) first, divide it up, and then go into each “module” and figure out how to write it. This separation of planning vs implementation works wonders because it allows you to focus on these two tasks independently. And to put it another way, the planning phase lets you see the bigger picture, whilst the implementation or writing phase lets you focus on the details of what you actually need to write and how to write it.
So next time you need to write anything that is somewhat involved—be it an email, training material or a manual—remember to divide and conquer.