I want to do my mothers taxes. How do I start a new return on my same login?

[mesquitebean]

You can't.   Each online account can only handle 1 Federal return per tax year.  If you try to prepare a second return in the same account, it will overwrite the first one, making it unrecoverable.

A separate account (separate User ID) is necessary for each Online return, and a separate fee (if applicable).   You can use the same email address in the Account Settings (Profile), if desired for notification purposes, but each User ID must be unique.  A User ID does not have to be an email address.

For future reference, the desktop products (CD/download) can handle multiple returns.

Please see this FAQ on how to start a second Online return:

https://ttlc.intuit.com/replies/3288574