I am a Recruiting Operations Officer at an ROTC program. You need to schedule a visit with whomever is my counterpart at the school with the ROTC program. Each program does things a little differently so the ROO can tell you the specifics of how it works at that program. SMP is essentially a 3-way agreement. You have to want to become an officer, your unit commander agrees to take you on as an ROTC SMP cadet, and the Professor of Military Science has to agree to the same thing. The last one is why visiting the program is so important. I do not recommend anyone to become an SMP cadet without meeting face to face. Each program looks at prospects in terms of being scholars, athletes, and leaders so you want to make sure you are strong in those areas. At my school, unless they are coming in as a junior, I recommend they try ROTC for at least a semester before they make a decision. That also gives the program cadre some time to assess your leadership potential. But again, how I do it will be different from other schools. So stop in an see the program's ROO ASAP. Good luck and PM me directly if you have other questions.
Apply to a college with an ROTC program and get accepted. Then as MAJ Chu recommended - start talking to the ROTC Department about SMP! If you already have college credits, and/or you are already in the USAR and have attended basic training, you may be able to enter ROTC in the "advanced Course" for Junior-Senior years (2 years) only. If you are starting from scratch, you can enter as a Freshmen, start ROTC, and then apply for SMP. Good luck!
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