You look at the last hiragana of the dictionary form of a Godan verb and apply the change:
Since "Pastakudasai" is a pedagogical label, the best papers analyze the underlying grammar of using past/perfective forms for requests.
This phenomenon is analyzed in linguistics as the interaction between (perfective/imperfective) and Modality (requests). When a past/perfective form ( -ta ) is used for a request, it implies that the action should be completed quickly or done once, whereas the -te form implies a continuing action.
The rule is based on the phrase: