From memory, this should be the full process:
1. Make a time machine backup of your Mac, or at least backup everything you need
2. Clone your backup onto another drive to be safe
3. Download Yosemite from the App Store (App Store > Purchases > Yosemite > Download)
4.
Follow Option 1 of this guide to make a bootable USB
5. Check your backups
6. Restart your Mac and hold 'alt' (option key)
7. Select the 'Install OS X Yosemite' USB from the list using arrow keys and hit enter
8. Once booted, choose Disk utility from the list and load it up
9. Select your main drive (should have 'Macintosh HD' nested below it), and click the Erase tab
10. Ensure the format is 'Mac OS Extended (Journaled)', and call it 'Macintosh HD'
11. Click 'Security options...' in the lower right, and move the slider
at least one notch to the right
12. Click 'Erase...' in the bottom right and say yes/ok to the warnings
13. Quit disk utility and then click 'Install OS X'
14. Follow the prompts to install Yosemite on 'Macintosh HD'
15. Try to avoid signing in with your Apple ID at any stage during first boot