The simplest advice is "tell 'em what you're gonna tell 'em, tell 'em, tell 'em what you told 'em."
I like to start from an extremely straightforward position on writing introductions and conclusions. I'm writing a paper now about the US healthcare system and my placeholder introduction paragraph is "The US Healthcare system is bad for X, Y, and Z reasons and should be changed." My placeholder conclusion is "Now that I have illustrated that the US Healthcare system is bad in X, Y, and Z, ways, I hope we have all learned something and take A, B, and C steps to change it."
Basically I write out the most basic thing I want to say in each paragraph and then embellish it. Sometimes this will actually lead to restructuring the paper a bit as I organize paragraphs to make sure that X, Y, and Z are in the proper order.
I have more trouble with conclusions than I do with almost any part of a project, but one of the things that has helped me with more academic-y texts is recognizing that if you've done your job properly the reader should know why you're making the argument you're making so you don't have to have a rabble-rousing, inspiring conclusion, you can functionally just say "Hope that clears things up! Here are the implications I want you to leave this paper with and my policy suggestions for the future."
Intros are a little easier for me because I just see them as scene setting. Treat it almost like an abstract, if that helps. "This paper is about this subject, here is my opinion on this subject, here is a brief summary of the evidence that supports my opinion on this subject. Here are some considerations to keep in mind, and here is why I think you should agree with my opinion."
Depending on the norms for the subject your intro can also include a brief history of the scholarship around that subject, biographical matter about a person under discussion, or a short explication of theory. I personally love multi-paragraph intros that spend a while getting me up to speed, but I also read literary criticism recreationally so I may be a bit biased. I would definitely say to find some field-specific papers that you liked and found useful to read and see how they constructed their introductions and conclusions and take some cues on structure from them. You can even go sentence-by-sentence and break down what each sentence is saying in the conclusion of a paper you liked ("As you can see from the previous paragraphs on SUBJECT, there is ample evidence of THESIS. We have responded to counter-arguments by addressing ISSUE and OTHER ISSUE. Our findings support THESIS, and you should agree for REASONS.")
Actually you know what that's my advice to everyone having trouble with intros and conclusions: find some intros and conclusions that you like and turn them into mad libs because that's basically what they are. That's a really good way to practice seeing what parts of your paper are unique (to fit into the blanks) and to figure out the structure of an academic intro or conclusion (the frustrating bit that is difficult to write).
