Start with interruptions
For a typical workweek, note the questions that come back to the owner: Can we quote this? Who handles that? Is this ready? Include what information the owner checks before answering. You are looking for repeatable decisions, not a transcript of every conversation.
Turn one decision into a usable rule
Fictional example: only the owner knows when a custom request needs a site visit. Record the conditions that trigger a visit, the information needed to decide, who can approve it, and what to do when the request falls outside the rule.
A useful entry has five parts: situation, information needed, normal decision, person responsible, and exception route. Include one worked example. Keep the owner's reasoning where it matters; a checklist without the reason can fail on the first unfamiliar case.
Separate a rule from a judgment call
Some decisions can be delegated within agreed limits. Others still need the owner or a qualified specialist. Write that boundary plainly. Documenting a decision does not automatically authorize someone else to make it.
Test a short absence
Have the backup person complete a safe example using only the notes and their permitted access. Record where they need help. Fix those gaps before producing a hundred-page manual nobody has used.
Start with the work that would stop if the owner were unavailable tomorrow. Assign someone to keep each instruction current. The aim is continuity—not turning the owner's entire brain into a folder.