Hold the knife firmly in one hand. Make sure that you hold it far enough from your body that you do not cut yourself when trying to close it.
Flip your wrist either to the left or right, depending on which hand you are holding it in. Release the opposite handle as you flip your wrist. For example, if you are holding the knife in your left hand, you will release the right handle as you flip your wrist.
Grab the handle as it spins over the top of the blade. Maintain your grip on the other half of the handle as well. Some butterfly knives have a latch at the bottom of the handle that you can fasten once you have closed it. This will prevent your from opening it accidentally.