Popup Next Button Help

Hi, looked around the forum and not seen an answer that I can follow (may be because I don’t fully understand it). How do I move forwards and backwards through whatever is showing in the RG records (using previous and next) buttons in the popup that is editing the task?
I saw a post about setting an index which may be the answer but how can i set this, and will it work when the RG is filtered/sorted by different users?

Thank you

Paul