Q: how can I revert to os x yosemite from el capitan

The only possible way without Time Machine is to erase and install through internet recovery.  This will install the original operating system that shipped with the computer.  You can upgrade back to Yosemite by downloading Yosemite through the Purchased tab in the App Store.

This will erase everything from the computer, make sure to backup any important data first.

How to reinstall OS X on your Mac - Apple Support

I would think creating a Yosemite Bootable USB installer should work unless I'm missing something.

One way: Create a bootable installer for OS X - Apple Support

You can also use DiskMaker X.

dialabrain wrote:

 

I would think creating a Yosemite Bootable USB installer should work unless I'm missing something.

Yes, but it would still require having to erase first.

Yep. A backup should be made. I do think it would be faster than doing an Internet Restore. I have a fairly fast connection and it's still slow.