If you've ignored our advice in other articles and installed Yosemite on the only volume on your only Mac, you must depend on the generosity of your friends. That means turning to a friend (who hasn't installed the Yosemite beta), begging to use his or her Mac, and downloading a copy of Mavericks from the Mac App Store using your account.
But even here, there's a hitch: If your only Mac is a newer Mac that shipped with Mavericks, you can't download the Mavericks installer from the Mac App Store--your Mac uses OS X Recovery to reinstall OS X. And because you updated to Yosemite, recovery mode now reinstalls Yosemite.
In this case--you've installed Yosemite on your only Mac, it's a newer Mac, and you don't have a backup that contains the Mavericks installer (or is a full, bootable clone of your pre-Yosemite installation)--your only option for getting a copy of the Mavericks installer is to rely on the generosity of friends. Specifically, you'll have to beg one of those friends to lend you their copy of the Mavericks installer.
Choose (or create) an install drive
In order to use the Mavericks installer, you must boot from a volume with Mavericks (or an earlier version of OS X) already installed. As mentioned above, if you boot from a Yosemite volume and attempt to run this installer, you'll be denied. So if you have a partitioned volume that can boot into either Mavericks or Yosemite, or an external volume that boots into Mavericks, start up from that Mavericks volume.
If you don't have such a volume, but you do have a copy of the Mavericks installer app, glom onto an 8GB-or-larger flash drive and follow the advice in Dan Frakes' seminal How to make a bootable Mavericks install drive. (This is different from his just released How to make a bootable OS X 10.10 Yosemite install drive.) You'll use the techniques described in the article to create a bootable Mavericks installer volume. Of the options outlined by Dan, I've found using DiskMaker X the easiest.
Wipe the Yosemite volume
When the Yosemite beta was installed on your Mac, the startup volume was changed in significant ways--so significant that if you attempt to install Mavericks over Yosemite, you'll find that the Mac doesn't behave properly when booted into Mavericks. For this reason, you must start clean by erasing the volume. Naturally, you should back up any data on the Yosemite volume before proceeding.
If you've booted from a Mavericks volume, you can erase your Yosemite volume by launching Disk Utility (/Applications/Utilities), selecting the Yosemite volume on the left side of the resulting window, choosing the Erase tab, and clicking the Erase button. (You needn't bother with the Partition tab or Partition options, because the volume is already formatted properly if it was running Yosemite.)
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