You should identify imperfect/preterite:
Preterite: Actions ended/finished. You can use this to tell a story.
Imperfect: An action from the past, but one that hasn't necessarily ended.
The use of ser/estar in the past is similar to how it is used in the present.
Okay, let me explain this:
"She was very pretty."
"Ella fue muy bonita." -> Preterite / ser, she was pretty but isn't pretty now.
"Ella era muy bonita." -> Imperfect / ser, she may or may not be pretty now.
"Ella estaba muy bonita." -> Preterite / estar, she was pretty at one specific period of time, for example, at a party.
"I was fat."
"Yo fui gordo." -> Preterite / ser, I was fat, but I am not fat anymore.
"Yo era gordo." -> Imperfect / ser, I am not talking about the present, I may or may not be fat now.
"Yo estuve gordo." -> Preterite / estar, I am talking about a concrete state, "In the 70s, I was fat." The action has ended, I am not fat anymore.
"Yo estaba gordo." -> Imperfect / estar, I am talking about a concrete state, such as, "In the 70's, I was fat." I am not expressing anything about the present.
"It was dark at the party."
"Estaba oscuro en la fiesta." -> Preterite / estar, I am talking about a story from the past that has ended or maybe the lights went on (so it is no longer dark).
"Estuvo oscuro en la fiesta." -> Imperfect / estar, I am not expressing anything. Maybe it is still dark or maybe it isn't.
"Era/fue oscuro en la fiesta." -> It doesn't make sense to use era/fue with oscuro; the quality of being light or dark is usually something that is in a fluctuating state as in, it may be dark now, but it won't be dark forever.
"She was furious."
"Ella estaba furiosa." -> Preterite / estar, she was upset.
"Ella estuvo furiosa." -> Imperfect / estar, she was upset (this usually is used in an early-past, so she may be furious still).
"Ella era/fue furiosa." -> Doesn't make sense, furious isn't an identity, it's a state, a fluctuating situation. Nobody is furious forever.
"He was a happy man."
"Él fue un hombre feliz." -> Preterite / ser, he lost the happiness somehow (perhaps he is now dead?).
"Él era un hombre feliz." -> Imperfect / ser, is he happy now? We don't know, but it is possible.
"Él estaba/estuvo un hombre feliz." -> Never! You shouldn't use "estar" with "un/una." Anyway, happy is an identity, not a state.
Editor's note: While it is true that "happy" can be considered an "identity," it can also be a fluctuating condition. For example, someone can be a happy person in general or just happy at a particular moment in time for a particular reason. But, as you can see from the discussion in this thread here, there appears to be a lack of consensus on whether or not one should use "ser" or "estar" with "feliz." This graph from Google's Ngram Viewer, further supports that "estoy feliz" is being used by some:
I did not do a thorough examination of these examples. In fact, I didn't conduct any at all, but while I have difficulty finding it natural that native Spanish speakers would have more reason to say I'm happy (as in I'm a happy person) over "I'm happy" (at this moment in time), I readily see a need to use "ser" for instances in which one might wish to express "I want to be happy" or "I want to learn to be happy." Typically, these last two sentiments are making reference to a permanent condition and not a singular, fleeting moment in time.