Instructional design is the discipline of deciding what somebody needs to be able to do, and then building the material and the sequence that gets them there. It is a design practice rather than a writing practice: the output happens to be a course, but the work is the analysis before it.
What the discipline actually covers
Four questions do most of the work. What should the person be able to do afterwards, stated as an action rather than as a topic. What do they already know. What is the smallest thing that closes the gap. How will you know whether it worked.
The last one is where most operational training quietly stops. Delivering the material and confirming attendance answers a different question from whether anybody can now do the thing.
The models, and what they are for
You will meet ADDIE (analysis, design, development, implementation, evaluation) and various iterative alternatives such as SAM. They are project structures, not theories of learning. Their value is that they force the analysis to happen before the writing and the evaluation to happen after the delivery, which is exactly the order most organisations reverse.
Treat them as checklists rather than methodologies. A model that adds three weeks to a procedure change you need on the floor on Monday has cost you more than it returned.
Where it earns its keep
Two things, mainly. Cutting scope, by separating what a person must know to do the job from what is merely true and in the manual. And sequencing, by putting the thing that makes everything else make sense first.
Both of those are why a well-designed five minute module can outperform a thorough forty minute one. The difference is not production quality, it is what was left out.
Nobody in your operation has this job title
Here is the practical point for a multi-site operation. You almost certainly do not have an instructional designer, you are not going to hire one, and your site managers are not going to become one. Advice that assumes otherwise is written for a different reader.
So the useful question is not how to acquire the discipline but how much of it can be built into the process instead. A defined structure that every module follows, so nobody designs from scratch. A default length, so scope has to be argued for rather than assumed. A knowledge check that is required rather than optional, which forces someone to state what the person should be able to do. A version on every record, so you can tell what was taught when.
None of that requires expertise. It requires the decisions to be made once, centrally, and then not re-made by every author. That is the honest version of instructional design for an operation that does not have one.
Bring one work instruction to a demo and we will show you what it looks like once those constraints are applied to it.