If you've come to this guide months after you initially installed Yosemite and would like to do so again — on the Mac you're using or another one you own — there are a couple of ways forward.
Just as you could before, you can boot into Yosemite's Recovery partition by holding down Command-R at startup, and in the OS X Utilities window that eventually appears, click Reinstall OS X, and then click Continue. This launches the OS X installer, which will download a fresh copy of Yosemite and install it on your startup volume.
Alternatively, if you no longer have the original Yosemite installer that you downloaded from the Mac App Store, you can get another copy by selecting App Store from the Apple menu, clicking on the Purchases link, and clicking Download next to the OS X Yosemite entry.
If the word Download doesn't appear next to the installer, it means that the App Store believes you have a working copy of the installer already on your Mac. You should locate that copy and if you don't want to use it — because it's now a couple of point-releases old, for example — either compress it or remove it from your drive. Once you do, you should be able to download a fresh copy from the Purchases page.
Welcome to Yosemite
You now have a working copy of Yosemite on your Mac. Although the installer has completed its job, there are a few things you'll want to do before moving on. They include the following:
Check for software updates: Although you just updated your apps before upgrading to Yosemite, check the App Store again to see if there are new updates that are available only to those Macs running Yosemite. Install any you want or need.
Set up a Time Machine backup: If you have another drive attached to your Mac you'll be asked if you'd like to use it with Time Machine. If that's the purpose of that drive, allow this to happen. If you don't have a backup strategy, this is a good time to devise one. The simplest way is through an external drive, but you can also back up over the network to a Time Capsule or some other kind of network storage. The data you save may be your own.
Add your printers: If you still apply ink to paper as part of your computing life, you'll want to be sure that your Mac is aware of the printers you use. Launch System Preferences, select Printers & Scanners, and see if your devices appear in the list to the left. If not, click the plus (+) button and add them. In some cases, printer drivers will be built in. In others, they'll need to be downloaded.
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