1. Download vSphere ESXi ISO from here: https://www.vmware.com/go/get-free-esxi
2. Download UNetbootin (Linux, Windows, Mac OS) or Rufus (Windows).
3. Start UNetbootin and choose:
   • Diskimage
   • ISO
   • Select the downloaded ESXi ISO
   • Type: USB Drive
   • Drive: Select your USB drive
   • Select OK
4. Boot your host off of this USB drive.

References

• http://www.satheesh.net/2011/08/29/how-to-make-a-bootable-vmware-vsphere-5-usb-installation-media/