Getting Teaching Positions
There are many paths to teaching, so what I did may not be what your uGSI did. However, my journey from student to 20Hr TA was about as "normal" as it gets. I took CS 61B in Spring 2019, during my freshman year at Berkeley. The summer after, I stayed in Berkeley to take summer classes, so I AIed for CS 61BL. I was hired to be a tutor in July for the Fall and from there I went from tutor to 8hr TA to 20hr TA. To maximize your chances of getting an interview, it's important to have AIed or taught otherwise in the past. We don't want TAs who may not be committed to a full semester of work. It also helps to have taken CS 370, since it requires 3 hours of tutoring a week which is great practice. Assuming you have the experience to be a TA, here are some tips on getting through the application process.
Write Good Responses
For most lower-division courses, there is a supplemental form that has a few short answer questions about how you teach. You should answer these questions thoroughly and insightfully. We try to be relatively blind to things like grades (though this isn't the case for all classes), so these responses are how you set yourself apart from other applicants. Write at least a paragraph and pull in details about yourself to convey what you believe in and what your teaching philosophy is. Don't ever blame the students if the question asks about why students may struggle in the class. Don't make the entire point of your essay that you loved taking the class. Don't talk about things that are irrelevant to teaching (like a dev internship) unless you are talking about how they would be useful to course staff.
Prepare for the Interview
There are a non-zero number of applicants who come to the interview with no preparation. This is your one shot to show us that you would be a good TA in practice. We don't care what you talk about unless you are wrong, so make sure you pick a topic you are comfortable with and can take questions on. Make materials ahead of time (these can be slides, things written on an iPad, even a dropbox document) and practice performing it in front of others until you have it down smooth. Make sure to time yourself, but don't worry about how much material you cover. What's important is that you don't seem confused or hesitant about what you are talking about. Visualize what kind of students would come to your discussion - have they watched lecture? Have they done the homework? Present as if you are talking to students who have a realistic grasp of the material.
Present Like a Real TA
During the actual presentation, the interviewers will usually act like real students. This means you should act like a real TA. If you have an idea of what kinds of students you are presenting to, mention it at the beginning so the interviewers can fit the role as well. Pause for questions after major concepts. Don't skip over things that an average student would be tripped up by. Talk with charisma (have inflection in your voice, be pleasant, etc.) and try to get audience participation as much as possible. Think about TAs you've had that were engaging in the past, and adapt their mannerisms to your personality.
Don't Stress Afterwards
After the interview, don't stress about what happened. Usually, there are not many TAs hired compared to the number of applications so there may be qualified candidates that didn't quite make the cut. You can always interview again the following semester!