Wash the cover in warm water and mild detergent to remove any dirt or grease that could interfere with proper bonding of any paint or decals you may apply.
To improve the plastic's ability to bond with either paint or glue, use fine-grit sandpaper to sand the areas of the plastic case that you plan on modifying.
Apply an acrylic paint to the case to make it a color that you enjoy. You can use a sprayer or a small brush, depending on the size of the case. Once the paint dries, apply a second coat for best results.
Add lettering or detail to the case with a small paintbrush and different colors of acrylic paint.
Apply stickers or decals to the case that suit your personal style.
Add glue to the case in places where you want additional decoration. Verify the glue is designed for plastic to ensure your decorations won't fall off.
Attach glitter, fake jewels, colored string, or any other decorative items you want, and wipe away any excess glue before it dries.