For other uses, Disk Utility is free and will do the job.
Before you start, you should back-up the drive you intend to partition. Better still, clone it using a tool like Carbon Copy Cloner. Assume the worst will happen and you'll lose all the data stored on the drive when you attempt to partition it. Make contingencies. A bootable clone will have you up and running again in no time.
Once you've cloned the drive and verified you can boot from it by restarting your Mac with the clone plugged in and selected as the Startup Disk in System Preferences, you're ready to begin the process of creating a new partition.
Unplug the disk with the clone on it and restart from your Mac's normal startup disk. When your Mac has restarted, press Command-Shift-U or select Utilities from the Go menu in the Finder. Double-click on Disk Utility.
In Disk Utility select your Mac's internal drive, making sure to click on the drive and not the volume beneath it. You should see two tabs below the toolbar: First Aid and Partition. Click Partition.
Click the '+' below Partition Layout to add another partition to the disk. You'll see the layout change to show the additional partition. You can now change the size of the partitions by dragging the line dividing them up or down, subject to a minimum size for the boot partition which is dependent on the data you currently have stored on it. When you've made them the size you want, click on the new partition and type a name for it in the 'Name:' box. Make sure the format is Mac OS X Extended (Journaled).
Once you've set the partition sizes and named the new partition, click Apply to finalise everything. Alternatively, if you change your mind or want to start again, click Revert instead.
Your Mac's main drive is now split into two volumes, with the new one empty. You can now install a different version of OS X on it, use it to install a beta of macOS, or keep it as a place to store data.
You can re-size the partitions at any time using the same process you used to create the new partition, but instead of clicking '+' just drag the partition bar up or down. The degree to which you can re-size will be dependent on the data stored on each partition.
When you've created your new partition, you can install an operating system on it by double-clicking the disk image (you can download an installer for your current version of OS X, or any other version you've previously acquired with your Apple ID from the Purchases section of the Mac App Store). Select the new partition as the location for the installation. Then follow the instructions to complete the installation. Once it's done you can reboot into the partition whenever you need to.
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