When I approach a doc set that needs a lot of work, I think of my job as building an onion. That is, creating successive layers of quality. As I build my layers I learn the doc set and the product, making the next stages possible.
The first layer is typically the result of a copy edit: remove typos and grammatical errors.
Next, work on consistency. Start to revise terminology and wording conventions (and build up a style guide). Ensure the writing is active voice, second person, present tense. Read the doc critically, thinking about the suitability of the content for the target reader.
At this stage research is required: talk to managers, product managers, developers, customer support, and customers. Look at how competitors handle similar areas. Read Wikipedia articles for background. Read the standards that your product is based on. Read analyst research for your market. And so on.
Revisit your terminology decisions, and start to formulate a clear picture of your target readers.
Create a list of problem areas in the doc set. Prioritize them.