The first step is to define purpose and scope of the task. This is often obvious. If the task is to lubricate a pump, the purpose is to lubricate and the scope will be the points that need to be addressed (grease bearings, check oil level, add or change oil).
If the task is more complex, say an annual inspection of a system, I will do research to define the scope. This may involve consulting previous work orders, manufacturers' manuals, engineering drawings, and/or consulting engineers or the manufacturer's representative.
The second step is to create an outline or rough draft of the procedure to be sure all points are covered.
Then, if at all possible, I will examine and photograph the equipment. This is done to aide in illustrating the steps and to identify any obstacles that will need to be addressed. Recently I was writing about rebuilding a pump. A special tool was supplied by the manufacturer to aid in removal of the impeller. The pump was mounted too close to the suction pipe for the special tool to be used. Providing a solution to this problem made the procedure not only more accurate, but essential to performing the task in the field.
Finally, the procedure is written in final form with steps illustrated to make sure the person performing the task has everything they need to do the job without having to take time to do research themselves.
For an example of a completed mechanical procedure go to www.tomperson.com/downloads/sample_maintenance_procedure.pdf.