Is it ok to copy/paste a section of a previous paper of mine into a new one?
Technically this is self-plagiarism and not OK. We had problems before where we published the 6th paper on a very similar topic and one sentence in our introduction was the same as in another of our papers (there are only so many ways to describe one thing) and it got flagged and we had to rewrite it, which in my personal opinion is just stupid.

However, if it's part of a methods section it is usually tolerated, since it doesn't make sense to rewrite technical statements all the time. For example in synthetic chemistry you would write something like


  All chemicals were purchased from Sigma Aldrich and used without further purification.


Which is fine to just copy/paste.