You can comment on works and add stickers by using the comment box below the post.
Comments are limited to a maximum of 140 characters.
The work's creator can post comments on their own work.
If the creator disables "Comments", other users will not be able to send stickers or comment on the work.
The work's creator can also delete all comments and stickers posted.
Furthermore, even if you're not the creator, you're able to delete your own comments and stickers.