Click on the + button. You’ll notice that the map of the single partition has split in two. This means that you have set up Disk Utility to add a partition on the startup drive.
To adjust the size of the partitions, click and drag on the dot between the partitions in the map. To see the actual capacity when you adjust the size, click on one of the partitions so that it has an outline. Then you’ll see the capacity in the Size box.
You can also select a partition and enter the desired capacity in the Size box.
To add another partition (for example, if you want to create three partitions), select a partition and click on the + button. Clicking on the — button removes the selected partition.
Note that in the partition with the blue shaded area that represents data, you can’t make that particular partition smaller than the space the data is occupying.
You also need to set the format for the new partition. Learn more about the format choices. In most cases, you'll select Mac OS Extended (Journaled).
Click on the Revert button any time you want to start over from the beginning.
In this example, a second partition of 100GB will be created.