Apple announced OS X Yosemite at its Worldwide Developers conference Monday, and among other features, the new version is ushering in the single biggest visual change to OS X in, well,. OS X's visual identity has evolved continuously since version 10.0, but always in baby steps. Reading John Siracusa's OS X reviews from the last decade-and-a-half , you can watch the old candy-blue "Aqua" design morph slowly into the more reserved, muted UI that Mavericks uses, but Yosemite is in many ways a clean, iOS-7-style break from that evolution.

The final version of the software won't be available for a few months, but Apple developers (and, for the first time ever, one million public beta testers) can download and install the first Yosemite beta version now. Developers will be used to the process of testing new OS X versions on their hardware, but those people in the public beta may be wading into uncharted waters.

This guide is for those people, the ones who want to test the new OS X beta without wrecking their Mac—you never want to put your data at risk by installing beta software on your main partition. This way, you can play with all the shiny new features without risking the comfortable, stable, non-beta OS X installation you need to get all your work done.

Create a new partition with Disk Utility

Fire up your Mac and launch Disk Utility, which can be found both in the Utilities folder and through a Spotlight search. It's going to display any internal and external disks connected to your system, as well as mounted CDs or DVDs (do you still use those?) and disk images. Select your Mac's main hard drive, which should be easy to spot—it will use a grey hard drive icon and will probably have a partition named "Macintosh HD" underneath it.

Now, select the Partition tab. If you don't see the Partition tab, make sure you've selected the disk itself (in my case, it's called "251 GB APPLE SSD SM0256F," though this will vary from Mac to Mac) and not your Macintosh HD partition.

Click the Plus button to add a new partition to your disk, and then decide how large you'd like that partition to be. I find that 30 to 40GB is more than enough for the operating system and my basic set of applications, but give yourself as much as you think you'll need. Give the partition a name that is easily distinguishable from your main system partition, so you don't accidentally install the beta to the wrong one later. I just used "OS X Test," which is pretty straightforward. The default format should be "Mac OS Extended (Journaled)," and you should leave it that way.

Now click Apply, and after churning away for several seconds you'll have a brand-new OS X partition.

What if I can’t create a new partition?

You may open Disk Utility to try out the steps above only to find that it can't create and resize partitions on your internal drive. This doesn't necessarily mean anything is wrong with your disk—several totally normal activities can render your partition table immovable, including setting up a Boot Camp partition or encrypting your disk with FileVault.

If you don't want to disable and then re-enable those features (easy enough for FileVault, a giant pain for Boot Camp), your best bet may be to use an external disk. 2012 and 2013 Macs with USB 3.0 ports can actually provide reasonable performance using USB flash drives or hard drives, although obviously it won't be as fast as an internal disk. Thunderbolt comes even closer to internal storage speeds, though it's much more expensive. USB 2.0 and FireWire ports will work, though those interfaces' lesser transfer speeds will slow things down quite a bit.

I like to use this 32GB SanDisk USB drive recommended by Tested.com a while back. 32GB is enough room for the OS and most important apps, and the drive performs pretty well. If you've got a larger USB hard drive, you can use the same process as above to open Disk Utility and create a second partition on the drive for your testing.

Installing OS X

All OS X installers, including the installers for beta versions, are now distributed via the Mac App store. We'll use the Mavericks installer here to avoid violating any NDAs for Yosemite, but the process should be similar for either operating system.

All you'll need to do is download the OS X installer from the Mac App Store and double-click it. At some point, you'll be asked which disk you'd like to install the operating system on. Click the "Show all disks" button and select your new test partition (or external drive), and the installer will do the rest.

If you have enough storage space, you could even use the Migration Assistant to transfer files from your other partition as you would if you were going from an "old" Mac to a "new" Mac, though this will eat up a lot of space, and you'll have to deal with keeping both partitions current.

Once the final version of Yosemite is released and you're ready to switch over to using it full time on your main partition, you can open up Disk Utility and use the minus button to remove the test partition you created. Either adjust the size of your main partition or click and drag it to regain the space you used for your test partition.