Small companies run on people who know how things work. It is efficient until it is the reason you cannot grow, and it becomes a crisis the first time one of those people leaves.
The test
Pick your most experienced person and ask what happens if they are unavailable for three weeks. If the honest answer involves calling them, the process is not documented — it is remembered.
Now ask how long a capable new hire takes to become useful. If the answer is months, the cost is not just onboarding time. It is every hire you did not make because training them was too disruptive.
Document the work, not the ideal
The common failure is writing the process as it ought to be. What is needed is the process as it is, including the workaround for the customer with the unusual terms and the check the experienced person performs without thinking about it.
That last category is the valuable one. Informal checks are usually the real quality controls, and they are exactly what disappears when the person does.
Have the person who does the job write it
Not a manager, and not a consultant working from an interview. The person doing the work writes the first version, in their own words, at the desk, while doing it. It will be untidy and correct, which is the right order.
A single page per process is enough: the trigger, the steps, the decisions and who to ask. Anything longer will not be read or kept current.
Prove it with a new person
The only real test is handing the document to someone who has not done the job and watching where they stop. Every stop is a gap. Fix it and the document is done — until the process changes, at which point whoever changes it updates the page.
Ten to fifteen processes usually cover most of a small company. That is a few weeks of part-time effort and it is what makes the next ten hires possible.