Listed below, you will find several lectures by IELTS IDP, that give pretty good insight into what you need to do to move up the rubric from different assessment criteria.

Moving Up the Rubric in Different Assessment Criteria

Moving from a Band 4 to Band 5

IELTS Speaking: Band 4 vs. Band 5 - Pronunciation

https://www.youtube.com/watch?v=O8cRjM5IuP8

Moving from a Band 6 to Band 7

IELTS Speaking: Band 6 vs. Band 7 - Fluency and Coherence

https://www.youtube.com/watch?v=syHuEUdnbkg

IELTS Speaking: Band 6 vs. Band 7 - Grammatical Range and Accuracy

https://www.youtube.com/watch?v=Qx_dBv3ahhE

IELTS Speaking: Band 6 vs. Band 7 - Lexical Resource

https://www.youtube.com/watch?v=mrOgbTKe47Q

IELTS Speaking: Band 6 vs. Band 7 - Pronunciation

https://www.youtube.com/watch?v=zTOFJNl5Oxw&t=101s

Moving from a Band 7 to Band 8

IELTS Speaking: Band 7 vs. Band 8 - Fluency and Coherence

https://www.youtube.com/watch?v=XwgskILvIfU

IELTS Speaking: Band 7 vs. Band 8 - Grammatical Range and Accuracy

https://www.youtube.com/watch?v=7HnmrjPz06I

IELTS Speaking: Band 7 vs. Band 8 - Lexical Resource

https://www.youtube.com/watch?v=W-EkenvTvyA

IELTS Speaking: Band 7 vs. Band 8 - Pronunciation

https://www.youtube.com/watch?v=2ANrFWj7LRA

IELTS Speaking: Bands 5 to 8 - Fluency and Coherence

https://www.youtube.com/watch?v=owNwjiGd-RU

IELTS Speaking: Band 4 vs. Band 5 - Fluency and Coherence

IELTS Speaking: Band 4 vs. Band 5 - Grammatical Range and Accuracy

IELTS Speaking: Band 4 vs. Band 5 - Lexical Resource