New Job, advice wanted on how to write documentation

So I've just got this new job working on a web application developed using the Yii 1.1 framework. I've been told the code and documentation isn't in the best state, and therefore my first task will be to write documentation on how this all fits together. can anyone advise me on how to approach this task? I've written plenty of documentation before, but its normally alongside developing the software and never on a scale this large.

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