Rinse the cup with warm water and dry it with a clean towel to ensure all dust or dirt is removed from it. This will make your finished product look a lot better.
Cut the cup into whatever shape you desire using a pair of scissors. Plastic cups are usually thin and are therefore easily cut even with dull scissors.
Color the plastic cup with either markers or paint. Paint will usually provide a thicker coating and a smoother look but markers are usable as well, depending on what you're making and what your resources are.
Apply glue to the cup so you can apply decorations to your creation.
Stick glitter and other decorative items to the cup as needed to further transform it. Yarn can also be wrapped around the cup to add color or texture and the glue will help hold it in place.
Add stickers and decals as needed. Save this for last after any paint and glue has dried for best results.