Flip the CD so that the reflective side is facing up. The reflective surface of most CDs is usually silver in color but some have a tinge of purple, green or blue.
Use puffy paint or 3D paint to draw raised lines on the CD. Metallic-colored puffy paint goes well with the CD's reflective surface.
Draw shapes like circles, hearts or stars. If you don't have puffy paint, try making a homemade version by mixing 1 part acrylic paint with 2 parts white glue. Find the instructions here.