Check these links, they should provide you a clear picture on how to teach the ed pronunciation to your students...
Simple Past Tense Pronunciation
Pronunciation of past simple verbs | Teaching English | British Council | BBC
English Pronunciation: How to Pronounce -ed in English | EnglishClub.com
Simple Past - Pronunciation of the ending -ed
Pronunciation - The final ed sound of the simple past tense of regular English verbs - e Learn English Language

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