What does an enlisted Army Reserve soldier have to do to become an officer?

You must slay one with a PT belt, and drink of their blood under the Harvest Moon--naked and holding a spatula--then you must recite these words EXACTLY: "Klaatu barada nikto"!

Do that, and the universe will convey onto you, your Commission in the United States Army Reserve.

Or ya go jail. I forget which.

Same thing any other officer candidate has to do. Get a Bachelors Degree and complete a commissioning source (ROTC is expedited for enlisted personnel 2 years rather than 4), you could also go to a service academy or complete OCS. Talk to a recruiter/retention NCO or ROTC recruiter to start the process.

If you go the ROTC route I highly recommend you get into the Simultaneous Membership Program (SMP). This program allows you to drill with your unit as a cadet and bumps your pay up to E-5. You'll be non-deployable so that you can finish your degree and ROTC commissioning requirements and will remain so until you complete a Branch BOLC. A big advantage of this is that besides getting some useful experience as a drilling cadet all of your Reserve time counts as time in service. I taught ROTC at two Texas universities and used to ask my cadets if they knew the difference between a new lieutenant with SMP and one without. The answer is about $500 more per month on active duty over a 2LT with zero time in service. As an Army Reservist your only option for OCS is the Federal program at Ft. Benning (Guard has State run programs). OCS rules now require a completed degree and has (ever changing) age restrictions. I highly recommend the ROTC route unless you can somehow get someone in your command to get you an appointment to the U.S. Military Academy (USMA) at West Point. I'm a state OCS graduate myself and do not recommend OCS unless that is the ONLY option you have. It's basically a hazing with little training value. Both the OCS option and USMA require major horse power from senior leaders and a lot of luck. If you can get USMA don't hesitate or even think about it, just say yes. I turned it down like an idiot in 1988 and have always regretted it. It was another 15 years before I was given another opportunity to commission. I have 18 years enlisted time and again, highly recommend that you don't wait until you're nearly 40 to pursue a career as an officer. Get your butt in school and get it done.