OS X Mavericks is available in the Mac App Store now, but it’s only an upgrade—but if you want to do a clean install, you’ll need to manually burn an installation flash drive. Here’s how to do it.
Download OS X Mavericks from the Mac App Store, if you haven’t already. If it tries to start the installation, just close it.
Insert your USB drive (you’ll need one that’s 8GB or larger) and open Disk Utility.
Select your drive in the sidebar and go to the Erase tab. Format the drive as “Mac OS Extended (Journaled)” and name the drive “Untitled.” (Note: if you already have a drive or partition named “Untitled” connected to your computer, name it something else and change the corresponding variable in step 5’s terminal command, or you might experience data loss!).
Click the Erase button and wait for Disk Utility to finish.
Close Disk Utility and open up a Terminal window. Copy and paste the command from this forum post into the Terminal.
sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Mavericks.app --nointeraction
Press Enter. The process should take about 20 minutes, so don’t cancel it or eject your USB drive while it’s doing its thing.
When it’s done, you should get a message stating the process is finished. Now, you can restart your computer, hold the Option key to access the boot menu, and select your new USB drive to install a clean copy of OS X Mavericks.
Check out the video above to see the process in action, and good luck! Check out our overview of Mavericks to get started with the OS—that is, if you want to upgrade at all.