You'll create an Apply Texas account at the start of senior year (use your personal email not school email)
There is a $75 non-refundable application fee for UT Austin (You may be able to get this waived through free/reduced lunch)
For Texas schools use Apply Texas, for California schools use UC Apply, for other schools use the Common App.
The following are mandatory to submit to UT Austin: Application fee, 1 Long Essay (600-750 Words), 3 Short Answers (300 Words each) (these are not hard cut offs, my long essay was 770 and my short answers ranged from 290-330), proof of TSI Readiness (Exemption with SAT: Math 530, Reading 480), and High School Transcript
Processing times for transcripts to be sent, received, and processed will be around a week or two long so act fast once senior year starts
The priority deadline is Nov. 1 while the regular deadline is Dec. 1 (I recommend the priority deadline to have your application in early and to receive your decision faster)
Start writing your essays during the summer before senior year (you can find the prompts here), these will require a lot of trial and error, try to get your peers or reach out to admissions coaches to revise your essays
For more information on UT Austin admissions use this blog
If you are an auto-admit (currently top 6%) and are certain you are attending UT Austin, pay the $50 housing fee as early as possible if you are wanting to dorm on campus (this will give you the first time slot come selection day in the following summer)
OFFICIAL BREAKDOWN:
1) Start writing your essays around June of your summer going into Senior year
2) Perfect your essays by the start of senior year
3) Create an Apply Texas Account (August)
3) Take initiative and work with Mrs. Honey (College & Career Counselor) to send your transcript to UT Austin after you have submitted your essays and application (with fee) on Apply Texas and have received your UT Austin "MyStatus" portal log in (3-7 Business days)
4) Begin applying to scholarships (even if the are not worth much) on scholarship domains such as bold.org or Scholarships.com (TIP: you are more likely to win the ones that require essays or video submissions, also set a profile picture for your account to increase your chances of winning)
5) Work with Mrs. Honey and your parent(s) on the FAFSA Application (Financial Aid)
6) Wait on your decision (some may get them at the end of November or December (very rare) but most will receive them at the end of January) and enjoy your time before entering college
7) Once you receive your "1st Choice Major" decision, your official UT Austin acceptance banners and packages will come in the mail within 2-4 weeks, Congratulations if you have made it and you can now take your signature "Gone to Texas" pictures on campus!
*Do not sit there and torment yourself by religiously checking your "MyStatus" or the "UT Austin Admissions Reddit" every day, those who are auto-admits will receive their official UT Austin acceptances on MyStatus within a few weeks of applying, be patient with your "1st Choice Major" decision and focus on scholarships/living out your senior year because checking every hour virtually does nothing