First, anyone who is interested in this should buy the books:
https://www.informit.com/store/latex-companion-parts-i-ii-3r...
I was certainly glad to, since the binding was breaking on my 2nd edition due to constant usage, and I was glad to have the option of buying hardcovers.
Second, work through a good tutorial, there are many, but I like: https://tug.ctan.org/info/lshort/english/lshort.pdf
Third, learn your way around CTAN, and https://tex.stackexchange.com/ --- for pretty much any problem you have, there should be a package or code snippet which addresses it, if not ask on SE or https://old.reddit.com/r/LaTeX/ after searching (and let folks know what terminology you used in your search), and consider using one of the nice well-documented documentstyles suited to your project --- Koma is good for Europeans, I like Memoir and not just because the first author was kind enough to consider my suggestions for the manual or implement some simplistic code I sent in (has anyone heard from Peter Wilson lately?)
Lastly, when working with macros and packages and so forth, conceptually divide them between semantic markup (this bit of code describes how this text/number should be represented and thought of) and appearance (this bit of code is necessary to get the text on the page at this pagination state in the best possible way) --- all of the latter macros should be defined twice, once as null ops in one package file, a second time redefined to actually do what is desired in a second package --- that way, the page can be set without them just by commenting out the second package. It's even better if all such macros are named in such a way that they can be easily commented out w/ a find-replace.
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