Over the years it has often been necessary to document the work I do on help desks and produce it in a form that can be used as a reference manual for new starters.
Various companies that I have worked for have been working toward ISO 9001 accreditation, and part of this involves having good documentation that can form the basis of the material that is delivered to the new help desker.
It can be hard to write training documentation on a subject that you know well, as many of the steps you perform are done almost subconsciously. The answer to the conundrum is testing. When you have documented a procedure, you should test it, preferably using a test subject who doesn't do the job regularly.
Whenever I have ignored this simple rule, I have found that a small but vital step has been omitted and the procedure doesn't work.
To a regular user, the act of pressing Enter is usually taken as read, but unless you know this — and you can't assume that everyone does — the procedure will not work.
You should consider that the whole purpose of writing out procedures is to provide a resource to be used in the event that the person who normally performs a task becomes unavailable. It has always seemed to me to be foolish to allow a situation to develop where knowledge is kept by only one person, but you would be shocked to know how often this happens.
The main reason for documentation is to provide a resource to enable each task to be completed in a uniform manner. Things like password resets, which may require a paper trail and some security checks, need to be done in a completely standardized way. You can get into a lot of arguments if one person manages to get a password reset on request and another has to go through a security procedure, involving getting a manager to request it in a formal manner.
You can't document everything; most of the work you do on the help desk involves soft skills, which have to be learned through experience. Anyone can learn from a book but being able to ask the right questions, to pacify an angry user, to extract bits of information that the user may not feel to be relevant, and to gain the user's confidence is the real skill of the help desk, and one that sadly seems to be lacking on some of the help desks I call on regularly.