Apple don’t have instructions for how to do a clean installation of OS X Yosemite, but it is exactly the same as the previous version of OS X – Mavericks. So if you want a clean fresh installation of Yosemite, just follow Apple’s instructions here of Mavericks”
https://support.apple.com/kb/PH14243?locale=en_US
A clean install deletes ALL your data and resets your computer back to what it was when it was new. If you are selling your computer this is a good idea. Make sure you copy your data across to your new computer first. (With time machine or migration assistant or both.)
