A common user question is ‘how do I install a package on platform X?’ There are a couple of hints of this on here, such as a simple ‘adjust TEXINPUTS’ question and an
apt-get install question, and there's a ‘should we consolidate “how to install” questions’ meta-question. But I can't find a high-rep general answer, and the meta-question, though it has a good answer drafted, doesn't seem to have made it to the main site in a few years.
Have I just failed to find such a question?
I thought there'd be masses of duplicates of this question, but on reflection I can see why (pace my first sentence) this might not be as common as I'd have guessed: most people probably install pretty complete LaTeX installations rather than a bare-bones+tlmgr installation; and very few people will install packages from author tarballs.
My particular motivation is that I pointed to a beta version of a package in an answer, whereupon the questioner (reasonably enough) asked ‘fine, but how do I install that?’ I almost pointed towards Robin's venerable UK TeX FAQ answer, but thought I could surely do better.