Unfortunately, Apple doesn't provide documentation on how to use their own tools. (Grr.) So (and remember this WILL DELETE ALL DATA ON THE DRIVE):
- Open Disk Utility
- Select the drive in the pane
- Click the partition tab
- Choose "2 partitions" in the pop-up
- Size them as desired
- Click "options" and make sure the partition type is "Apple Partition Map"
- Click Partition
That should do it.