Motivation (can be skipped): I interned in LA two years ago. That year's SIGGRAPH happened to be held in LA as well. The full conference registration was quite expensive, so I got an exhibits registration and had a very brief taste of the conference.
This year, SIGGRAPH was back in LA. Since student volunteers get full conference registrations, I applied to be one and got the position.
Before the conference, I googled about what it was like being a student volunteer and found a few discussions on Reddit [1], and some blog posts from several years ago [2] [3] [4] [5] [6] [7] [8]. That's not a lot of resources. So I thought I would write about the (probably most up-to-date) experience of student volunteers.

How to apply:
Search for "SIGGRAPH {Year} Student Volunteer" and you can find a webpage with the relevant info. Right now, you can sign up to be notified when SIGGRAPH 2020's SV application goes live.
You can either apply to be a Student Volunteer or to be a Team Leader. The application can be accessed through an online portal. It consists of basic personal info questions and some short essay questions; the TL application has additional sections.

Timeline:
Applications open in {Year-1} Nov/Dec and close in {Year} Jan/Feb. Decisions and travel funds availability are released in {Year} Apr/May. Volunteer schedules are available in {Year} Jul (1-2 weeks before the conference), and then shift swaps start between volunteers using a Google spreadsheet. Volunteers arrive one day before the conference starts to attend the orientation.

Main Perks:
Full conference registration, Computer Animation Festival ticket and Reception ticket, 1-year ACM student membership, 1-year digital subscription of Animation Magazine, SV-specific sessions from sponsors, free food in the SV office, SV raffle.

General Q&A: I came up with a few questions but would be happy to answer more; please reach me through email.

Q: How many hours do SVs work?
A: This year it's up to 30 hours, and I had 26 hours. For SIGGRAPH Asia, it could be fewer. Most of the shifts involve standing and your feet will complain. But you can do this!

Q: How hard is it to swap shifts?
A: Pretty hard but possible. I contacted ~30 people before I found someone to swap with. Be proactive in this process and you'll be okay.

Q: Who are the student volunteers?
A: It's a diverse group: from college freshmen to Ph.D. students, from animation artists to graphics researchers, of different nationalities, races, and gender identities.

General Tips:

You will be asked to fill out a questionnaire about your interests after you accept the volunteer position, but your shifts won't necessarily reflect your choices.
Don't expect a lot of free time because your schedule will be quite packed. If you want to get more free time during the conference, definitely opt-in for pre/post-conference shifts in the questionnaire. But still, take advantage of your shifts because you might have some great encounters.
Your shifts may overlap with some SV-specific events. Fortunately, your fellow SVs are there to help. At least this year, some people organized a Google Doc for note-taking. I'm sure others can do the same in the future.

Personal Reflections:

It's my first time attending SIGGRAPH for its entirety. The past week was enriching and inspiring. I'm so happy and grateful that I got to meet many people whom I admire and to reunite with friends and mentors. These memories become part of my motivation.
It's my second time in LA. LA didn't leave me a great first impression two years ago. But maybe because I stayed in DTLA and didn't have to travel very far, I found it okay this time.
To be honest, I feel being a student volunteer is not worth it. I couldn't make it to the technical papers sessions I'm interested in; nor did I have enough time to visit the experience hall. Although there are plenty of opportunities to network with people in the industry, meet fellow students who aspire to join the industry, and to get a resume/reel review, they are not very relevant to a CS Ph.D. student. But if no one is sponsoring you to go to the conference, being an SV is worth considering.
Highlights: had some nice conversations with contributors when I worked the Emerging Technologies shifts; had a lab group photo at the siggraph sign; found all 25 sigglets in the ScavengeAR game; won a little prize in the SV raffle; more photos.