Most advice about writing one is about tone, and tone is the least of it. What decides whether a KB article works is its shape, which is a question as a title and the answer in the first paragraph, and what decides whether it is still worth having in a year is five fields that have nothing to do with the prose at all.
Title it as the question, in the asker's words
How do I get access to the billing system, not Billing system access procedure. The title is what search matches and what a reader scans, and it should use the words of somebody who does not yet know the answer. Internal vocabulary goes in the body, where search will still find it and where it will not stop anybody recognising the page.
Answer in the first paragraph, then qualify
A reader who gets the answer in the first two lines and leaves has been served perfectly. One who has to read four paragraphs of background gives up and asks a colleague, which is the cost the base exists to remove. Put the exceptions and the reasoning underneath, where the person who needs them will find them.
One question per article
An article that answers three related questions ranks for none of them cleanly and is impossible to review, because its three answers change on three different clocks. Splitting is nearly always right and the instinct to consolidate is nearly always the writer's convenience rather than the reader's.
The five fields
An owner, the date somebody last confirmed it was true, the review interval, the state (current, unverified, archived) and where the underlying authority lives if there is one. None of these is prose and all five are what separates a base that ages gracefully from one that has to be audited by hand two years in.
Keep the screenshots few and dated
Screenshots go stale faster than words and they are the most expensive part of a review, because updating one means reproducing the state it was taken in. Use them where the interface is genuinely hard to describe and nowhere else, and expect them to be the reason an article gets retired rather than refreshed.
What an article is not
It is not a policy, which needs an acknowledgement; not a process document, which describes work moving between people; and not a training module, which is sequenced and assessed. Each of those has an owner and a review cycle of its own, and pulling them into the base is how a base stops being searchable.
Questions people ask about kb article
How long should a KB article be?
As long as the answer, which is usually shorter than people expect. A page of two hundred words that answers the question completely is a better article than a thousand-word treatment of the subject around it.
Who should write it?
The person who already answers the question, in a draft, with an editor passing over it for the title and the shape. An article written by somebody who does not do the work reads well and gets the exceptions wrong.
Should the article carry its own date?
It should carry the date somebody confirmed it was true, which is not the date it was last edited. An edit date tells a reader that a typo was fixed; a confirmation date tells them somebody stood behind the content.
What about articles that are mostly a link?
Keep them if the link is hard to find and say in one line what the reader will get when they follow it. What to avoid is the article that is a link and nothing else, because it adds a hop and answers nothing.