At the paper proofing stage, you will be sent a PDF version of your paper to correct any typesetting errors. If you are unsure how to leave corrections on a PDF, then please watch this 5 minute video which is a get a comprehensive guide to all the commenting options in Acrobat DC, including comments, drawing tools, stamps, and more.

Link to video: https://helpx.adobe.com/uk/acrobat/how-to/add-pdf-comment.html