If you haven't made comments in a Google Doc before, it really is very easy. I'm going to show the instructions for Google Docs, but the process is essentially the same in Sheets and Slides as well.
When you have a document open, place the cursor where you'd like the comment to appear in the text. It normally highlights the nearest word. You can also highlight a specific word or section of the text that you wish to comment on. Then click the "Comments" button on the upper right hand corner of the window to open the comments thread. You will see the options for Notifications and Comment. Click on Comment.
Then it will pop up a little window for you to type your comment. After you type what you'd like to say, click the blue "Comment" button to submit your comment.
Then the comment will appear to the right of the document. All the comments will show up in that right side pane for the students. If they click the highlighted portion of the text, the comment that goes with it will be emphasized. If your students have email with their Google Apps account, they will get an email letting them know that you commented on their document.
If they want to reply to your comment, it gives them a little reply box. You can also type into the reply box to add to your comment. This creates a little conversation. You will get an email notification letting you know the student replied to your comment.
My students love checking their comments and responding back. Even if it is just to say "ok". Once they are finished with the comment, they can click "Resolve". The comment will close, and appear to go away, but you can access it again by clicking "Comments" on the upper right hand corner of the page.
With my younger students, I try to do a face to face conference about their writing and I leave little comments to remind them of what we talked about. If the comments are very simple and straight forward, I can leave the comment without the conference. When I'm at home at night grading their writing, I try to make the comment as clear as possible, but I might ask them to come and see me in the text of the comment so I don't forget to meet with them the following day. I love it, the students love it, and I love how their little personalities come out when they respond to the comments.
Stop by again soon for more handy tips!