Ground Effect is a topic discussed in a few of the PGIs found on this website. You can find in Exercise 07 – Climbing. However, if you are looking for a quick reference for yourself as a student or as an instructor to help your student you can find a great visual example of ground effect and how induced drag decreses as you become closer to the ground and how it increase as you become farther away.
The resource can be found at rodmachado.com. It may or may not still be available. You can click on the image or use this link to take you to the ground effect learning tool.