Here's the Q&A from the Travis County Election Clerk.
Q: Can you vote a straight party ticket and then make an exception on one or more races?
It works like entering a high score on a video game. You rotate the wheel through the selections and hit ENTER to select. The first page has a straight ticket selection that marks all of the rest of the pages, and then you can scroll through them and modify individual races.
When you submit your ballot, a little flag appears on the screen (and if you voted Republican, your selections are electronically discarded.)
;-P