Use How to get old versions of macOS to download an installer for OS X Yosemite while running on Yosemite. Run the installer and it will install the full installer to /Applications/Install OS X Yosemite.app

Use the command line from How to create a bootable installer for macOS substitution El\ Capitan for Yosemite in the El Capitan instructions.

Hold down option while powering on the Mac to get the option to boot from USB Recovery.