The title pretty much explains everything in this step. Just cut the bottom of the cup with a craft knife in which ever way you like. I can even be wavy!
Step 2: Design it
If you need to, use a pencil to draw in marks of where you want to cut. If you do so, do it lightly so that you can erase it at the end. Cut out your design with a craft knife. Creativity is encouraged. when you cut, go slowly, or you will be guarantied to make a mistake or a rip.
Step 3: Done!
Put your finished design upside-down on the table with a lighted candle under it. Make sure the candle is in the smack center so it won't burn anything up. Turn off the lights.