Here's how: Paint pumpkins and gourds white or black, leaving stems natural (or don't paint, if you prefer). Trace a few bat templates (download them below) on each pumpkin. Fill in bat outlines in opposite colors: black bats on white pumpkins and vice versa.
Tip: Enlarge or reduce template to desired size. Cut out template on a square of contact paper using an X-Acto knife. Stick the template to the pumpkin and trace with a grease pencil.
Get the bat template here