Draw the Batman symbol or any other shape you prefer, then have this marked by the Batman sign. For a more authentic Bat-shaped boomerang, use the actual Batman symbol as your project's shape. To do this, draw arches within the interior curve of a crescent. These should make edges that resemble a bat's wing. You can always check online sources for an accurate shape for your Batarang, then use it as guide. In terms of size, it's up to you how big you want the Batarang to be. As a benchmark sample, you can use a size of 4 inches by 7 inches for this Bat-shaped boomerang.
Cut out your Batarang's shape using a pair of scissors or cutter.
Cover the work-in-progress Batarang with tin foil to make it look like it's made of metal, then put a black Batman symbol somewhere on the Batarang. Alternatively, you can paint the piece with black paint instead of merely putting a Batman symbol on it as an identifiable mark.