In the App Store, log in to your Apple ID, search for Mac OS, and download the full version of the Yosemite installation package.
For larger ones, it may take two or three hours.
After the download is complete, it is located within the finder>app.
Don't worry about it, back up the files you need.
U disk or mobile hard disk larger than 8g.
The following is the content of Apple's official email steps.
Create a Bootable OS X Startup Disk - for Yosemite
1. First,
Guests need to prepare a U disk or external hard disk with a space of 8G or more, (or the system separates an 8G disk for use)
And partition and erase through DiskUtility,
Make sure that the partition method used by the U disk and external hard disk is: GUID, and the format is: Mac Journaled ,
Partition name: MyVolume (pay attention to case) set to the same name
2. You need to download the Yosemite installer from the Mac Apps store and store it in the default path.
(The default path is: Finder->Applications)
3. Paste the following commands through the terminal and enter the administrator's password
(Please note that there will be no display during the password input process, the password is displayed as an origin)
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app
4. The terminal will display a progress bar, as long as you see the word "Finished", it means the creation has been successful! (The actual installation is done)
Then restart the computer, and at the same time when you hear the sound of Dang, hold down the option key and select the Mac OS installer to start.
Then use a utility to erase the hard drive and install Mac OS on the hard drive.