Start with what gets asked internally
For two weeks, every time an agent asks a colleague a question, write it down. That list is your internal documentation backlog, ranked by real demand rather than by guesswork.
Internal articles, not a wiki nobody opens
Put internal procedures in the same knowledge base as customer articles, marked internal. Agents search one place, and the search already works.
Write the procedure, not the philosophy
Numbered steps, exact screen names, exact wording where it matters, and what to do when it fails. Context can be one sentence at the top.
Assign owners and review dates
Undated documentation decays into folklore. Each article gets an owner and a review date, and an out-of-date article is deleted rather than left to mislead.
The test
Could a competent new agent handle your third-most-common request using only the documentation? If not, that is the next thing to write.