“We’re” is a contraction of the phrase “we are”: the apostrophe stands for the omitted letter A. “Were” is simply a plural past-tense form of the verb “are.” To talk about something happening now or in the future, use “we’re”; but to talk about something in the past, use “were.” If you can’t substitute “we are” for the word you’ve written, omit the apostrophe.

“We were going to go to the party as a prince and princess, but Derek cut himself shaving, so we’re going instead as a vampire and her victim.”