IIRC and it's a bit hazy to be honest, he tried a coup, failed, was given Chancellor (head of the Parliment) then faked an attack and used it to grasp power over the Parliment then he instantly disallowed any opposite political parties. Bang, dictator.
Like I said the details are a touch hazy since it's been a long time since I've read anything on that period prior to the war.
Pretty much got all the main details. He was democratically elected; but the party never got more than 37% of the vote before the elections were openly rigged.
Another factor among many to bear in mind was the Weimar Republic was a complete basket case. Very susceptible to a strong leader taking control.