It took me a long time to figure out that I can write the best work instructions, the most articulate, clearly phrased and comprehensive writing and no one might understand it...
When writing a work instruction you want to make sure you take your target audience into account. Ask for input and listen to that input. It still needs to be professional and understandable but it can also include flowcharts, diagrams, pictures, and other information that make it simple yet effective.
It's a good idea to start with asking the operators on the floor to create one originally then take that and improve on it; through formating, cleaning up the text and wording, intergrating pictures or flow diagrams where it makes sense. It's a skill to communicate effectively with the spoken word; the same goes with the written word.