I was advised to write it as if you're describing the story to a friend (though obviously a lot tighter with the writing), e.g. first this happens, then this happens, we know that that happened in the past, so it ends with this happening, i.e present tense with a bit of past tense for relevant events before the scope of the story. You would never have that kind of conversation in the past tense unless you were describing real events.
T.