Which existing methodologies could be referred for supporting a PIA process ?

There a few posts in these pages addressing the topic. You can use the find tool and “DPIA” to have a list including: 
https://blog.europrivacy.org/2017/05/15/article-29-data-protection-working-party-guidelines-on-data-protection-impact-assessment-dpia/
https://blog.europrivacy.org/2017/05/03/guidelines-dpia-for-whom-for-what/
https://blog.europrivacy.org/it/2017/04/14/guidelines-data-protection-impact-assessment/
 
Also check this link:
http://ec.europa.eu/newsroom/document.cfm?doc_id=44137