Google Doc Help

Please choose the suggesting mode in the top right 
You can then make editing changes to the text and it will automatically track your suggestions, which I can then accept or reject. 
If you wish to comment, select the word or section you want to comment on, and then move your cursor to the right margin and a comment box will appear.  
You can then “talk to me” by making a comment (rather than a change).  

Don’t forget to click the “comment” button to save your comment.