Although not specifically designed for grooming pubic hair, regular hair clippers will do the trick. Use an attachment with the desired length you want(for example ¼"). Turn it on and run over your hair in the opposite direction of growth. If the attachments don't read with an actual length, you can estimate. The thicker the teeth are on the attachment, the longer the hair will be left. Warning, using the clippers without an attachment straight on skin doesn't make for a very comfortable shave in your bikini zone.
If you don't already own a pair, I wouldn't recommend purchasing for this reason. Regular clippers are large, heavy and more difficult to navigate on the body. Instead I would invest in a product for the same amount of money that's especially made for personal body grooming. Learn more about the options for both men and women in:How To Trim Pubic Hair