If the student is already logged in and using Write & Improve when you add or change some feedback, they will have to refresh their page or log out and in again to be able to see it.
Next time your student logs in to their account, they will have a badge in their sidebar next to the 'My writing' link under 'Progress'. The number tells them how many pieces of feedback they have that they have not yet seen.
When they click to see their writing, they will see a label against the answer for which feedback has been added.
When they click to see the answer, they will see the feedback and grade underneath the annotated text.