My great uncle used to tell me that you should never do anything for the first time. I've stress-tested this assumption, and I've found it to be a very understandable motto. He didn't really live by this creed, and I haven't either, but it bears remembering that the first few gos at anything are realistically going to be pretty rubbish.
That said, it's definitely easier to learn how to do something if you understand why you're trying to do it. When I understand the purpose of something, I have a kind of mental scaffolding upon which to hang other concepts. It's a bit like knowing the major roads through a city, or the very basics of cooking - you can roughly find your way through most things.
The question, then, is how do you put up that mental scaffolding? How do you do something for the first time? I'm working through this for myself, but here's where I am at the moment.
I'm going to take the example of writing a cover letter, because they're actually quite useful.
The cover letter is like a tour guide for your resume, picking out the highlights and crafting a story about you and your candidacy. Once the reviewer knows the story, they can refer back to specific elements, should they so choose. The cover letter can narratively tie together a CV and explain why it fits the job description, especially if the CV by itself struggles to make that sell.
I lecture at a university; as such, I have occasion to read a lot of cover letters. Cover letters submitted by students looking to join a programme. Cover letters submitted by academics looking for jobs. Cover letters drafted and re-drafted, seemingly forever, by myself and my collaborators as we scrounge about for funding. I also write a lot of references, which are very much like the cover letters except that they're about someone else.
I cannot tell you how much easier I have found it to write such letters after having sat on the opposite side of the table for a single job posting.
When I was writing my first cover letters, I wasn't thinking about the person who would be reading them. I knew that theoretically someone would, of course, and that oracular figure would return some verdict on my ~worthiness - but I never asked myself what it was that the reader was doing outside of how it impacted me. I never asked what was in it for them.
Having now been forced to read many more cover letters than any sane person could endure, I might approach writing a cover letter as follows:
The zeroth order point is that you need to have a general sense of what it means to do the job. What does the hirer want? Your letter's job is to convince the reader that you can do the job and that you won't be utterly intolerable as a colleague. You don't want to give the reader the impression that you don't understand the job - that's worrying, and might cause problems for them down the line. You also don't want to come across as unreliable, unprofessional, or any kind of liability. "I'm so hardworking that I snuck into the lab to run experiments by myself after hours" is the kind of thing that might sound impressive to you, but absolutely terrifying to your potential employer. *You have to either know or research what it means to do the job well*.
A complementary point has to do not with the ends, but the means of hiring. It is extremely helpful to know how hiring works in your target field. What I mean by this is, what even is the hiring process? How many people tend to be applying for any given job? What are you expected to include in your application? Again - you almost definitely want to research this before you apply.
At my university, we'll get easily 70 candidates for a part-time teaching role. We can't interview that many people, so we'll go through and discard any applicants who don't have essential features (eg qualifications or certifications). Then each interviewer will rank all of the remaining candidates against the set of qualities laid out in the job description. The "shortlist" for interview will be drawn from this ranked list, with the interviewers having to agree who's on the shortlist. You want to be aware of the process because it can help you see where you might run into trouble - and it can help you understand the importance of making your case extremely clear. You don't want the tired reviewer to miss the bit where you wrote that you actually DID have a qualification.
With that underlying foundation of knowledge, you'll finally actually start writing! Go through the job posting and highlight - literally, highlight them - the exact words the hiring person is asking for. You're going to use that to feed them the information they need.
Next, write out the things you highlighted, and for each of these points list out things from your CV that you could cite to back up your claim that you have that quality. Some things might fall into multiple categories, which is great because that means you have a narrative. Don't lose or change the phrasing in subsequent drafts! Remember your very tired reader, and help them search within the PDF for the information.
The "essential" and "desirable" characteristics, now with bonus evidence that calls attention to the parts of the CV you want to reader to look at, are the bones of the cover letter. Some of them naturally go together - for example, any kind of work with students, or any kind of work with committees. A lot of the structure can be emergent, and you can experiment with configuring the quality/evidence points in different ways before you start to join the parts together with actual sentences. Play with the skeleton before you decide how it works best.
In academic jobs in particular, there's another step which is often overlooked: you should explicitly discuss the way that you're going to fit into the university. Why do you want to go to *that* university (Hint: please don't say that you like the city where it's located - please?)? Who do you want to work with, and how do you think that your work would be compatible with theirs? If someone is hiring, it's because there's a need and a gap. Why do you want to plug that gap? What are you planning to do, once you're there? Are there research groups or trainings or facilities that are particularly interesting to you? If the person reading your letter can see that you're actually familiar with their work, and believes that you sincerely are interested in working with them, they can directly understand how hiring you would benefit them. It also shows, again, that you understand the job and what's expected.
Tell that story, then: why, specifically, are you interested in the role for which you're applying? Why should the reader believe that you can do the work? What is your vision for what you're going to do - and again, do you have experience doing something like this? What would you do if you got the job, outside of the explicit duties (eg institutional citizenship stuff, supervision, etc)?
It all seems very straightforward to me now, and a bit silly to write down. It's advice I keep having to give, however. May it help someone else as they, bravely or foolishly, do something like writing a cover letter for the first time.