Where do I indicate on a return that I am preparing it on someone else's behalf?

Only a paid preparer needs to sign stating that they prepared the return.  You do not need to sign the return or record that you prepared it for your mom.  She needs to sign the return (if filing a paper copy) or you need to fill in her ID information.

You do not enter anything about yourself on your mother's tax return.   Family members often prepare returns for other members of their families using TurboTax.  Assuming you have your mother's permission to file the return for her, you just file it as "self-prepared."