One of the services I provide is writing product manuals.
These often either contain too little detail or too much detail.
If you have a product that requires a manual, and you wish to write it in-house, I would suggest the following:
Have it written by someone who is not very familiar with the product, because experts will subconsciously deal with any problematic aspects automatically.
Consider imposing what I call the 5 minute rule, which is: what is the minimal amount of information I need in order to achieve something meaningful and useful in just five minutes.
Have the guide tested by someone else who is not familiar with the product.
Finally, put the manual in a sensible place. A few years ago I bought a printer that came with instructions on how to open the box. Unfortunately, they were inside the box.