When I write something that could be clarified or improved by a reference or a quote from an earlier post of mine, here or elsewhere, what is the preferred way to do so?
First, take a step back
If you want to copy content from an earlier answer, maybe the question should be closed as a duplicate.
If it is not a duplicate, are you sure that the content is really helpful to the asker, and not just remotely related (in which case a comment with a link would suffice)? We prefer quality over quantity.
Be clear that it is not original content
Provide a link, and put the relevant text in a quote.
Like so. (ref)
Don't copy too much
Only quote the most important part, and provide a link so that interested readers may click through. Don't clutter your new post with only semi-related stuff.