Use them to indicate ownership and use it to indicate omission of letters as in a contraction. Do not use them to indicate plurals. You should mind your Ps and Qs, not your P’s and Q’s. The Civil War was in the 1800s, not the 1800’s.
In the comments to the linked post, someone suggested that the lack of an apostrophe before “till” is an exception to the rule about omitted letters. But it is not. “Till” is a standalone word that can substitute for “until” all by itself. It may have derived from “until,” but it is not merely a shortened form of the longer word.