How to fix Google Shopping GTIN error for antique coins?

Thanks a lot for the links. After I had recognized that 50% of my products had been not approved my google merchent, I had implemented the changes/ rules from your linked youtube video.

However, now I wanted to do this:

https://feedarmy.com/kb/how-to-reprocess-content-api-in-google-merchant-center/

But I dont understand this part:

  1. Open the file using your favorite spreadsheet editor and copy the ID column (it can also be called item_id, if so change the header from item_id to id).
  2. Go to Google Sheets
  3. Paste the ID column contents

What do I have to do exactly? For me I use excel. For CVS files I know that it should be imported as data. and then?