If you choose to proceed without transferring data you'll be asked to sign in with your Apple ID (you can choose not to as well as choose to create an Apple ID if you don't have one). You're then offered the opportunity to share your location with iCloud so that your Mac can be tracked by iCloud's Find My Mac service (part of the Find My iPhone scheme found on iOS devices). If you've enabled two-step verification for your Apple ID, you'll next be asked to select an authorized device on which to receive a verification code that allows you to proceed with the installation. Enter that code when you receive it.
A Terms and Conditions screen appears next. You must agree to it (twice), otherwise you can't proceed.
You're nearly done. If you haven't already set up an account on this volume (meaning that you're performing a clean installation rather than updating an existing account) you'll be asked to create such an account. By default Yosemite allows you to use your Apple ID as your login but you can disable this option and do things the old-fashioned way: Create an account name and enter and verify a password.
You can optionally add a hint in the appropriate field. Also enabled by default are the Allow my Apple ID to reset this password and Set time zone based on current location options. Switch these off if you like and, if you want to, double-click on the icon that the installer has assigned to your name and choose a different picture. (Or select Camera and take a picture with your Mac's camera and use it as your icon.) Click Continue and the Mac will set about creating the account.
You're now asked to set up iCloud Keychain, which holds passwords and account settings. You can set it up now and verify it, or skip this step and do it later. I prefer to do it from the get-go so that my preferred web sites and passwords are waiting for me when I launch Safari.
Finally, choose whether to send diagnostic data to Apple and/or crash data to third-party developers, click Continue, and your Mac moves to the Yosemite desktop, ready to use.
If you install over an existing copy of the Mac OS, you'll encounter far fewer steps. Specifically, when installing over Mavericks you'll be asked to choose a Wi-Fi network, sign in with your Apple ID, choose to use Find My Mac (and verify your identity if you've set up two-step verification), agree to the terms and conditions, and choose whether to share diagnostics and usage data with Apple.
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