Learn to write instructions clearly.
There are a lot of jobs in which you will have to tell other people how to do things. These include teaching, management, and many technical fields. When a programmer writes API documentation, or a scientist writes up a design for an experiment, they are giving instructions. When a manager explains to a new hire what they are expected to do on the job, they are giving instructions.
Ordinary good writing skills are part of this. If you write sentences with ambiguous grammar, or use pronouns without clear referents, your reader is going to have a hard time. ("Separate the red part from the blue part. Then throw it in the fire." Which part gets thrown in the fire?)
But that's just part of it. Instructions have to be relevant to what the reader is trying to do. API documentation shouldn't discuss internal details that the user can't interact with. Instructions have to be accurate. If your manager tells you that the job requires doing X, Y, and Z, but then you get dinged on your review for spending any time doing Y, something is amiss. Instructions have to be adequately complete. If a scientific paper says "we centrifuged the mixture for 10 minutes at a moderate speed," that's not gonna fly unless "moderate speed" means something very specific.
I strongly agree, but which writing textbooks, courses, or other material does the best job covering this material?
I was looking at a discussion of what should be in a college curriculum, and as such discussions seem to go, there was a big list of things everyone should study, and some political claims about what's being offered but shouldn't be.
Instead, what do you wish you'd studied in college? What do you wish other people had studied in college? On the latter, do you think everyone should have studied it, or do you just wish more people knew about it? Approximately what percentage of people?
Of course, this doesn't have to be limited to college. People could learn the same things earlier or later.