If I want to join SMP, how do I do it?

It doesn’t matter when you go. The question you need tonansewr yourself is: do I need to? Is that going to affect my timeline (you’ll add 6-8 months to your graduation date)? Will your weekends and annual training be an inconvenience/conflict with your academics? Relevant experience, there is not much experience (in the great scheme of things) to gain in years as a drilling reservist/NG, either enlisted or SMP. Although you’II will meet great folks!

You can enlist in the Guard and go to basic, AIT and drill with a unit for your first two years of college. When you near your Jr year of college you sign a SMP contract. You don't need to join the guard or reserve until that point but if you join earlier you can be paid the whole time.

Are you a Freshman?
If I were a freshman, I would join the National Guard, get BCT/AIT out of the way, which you can do split option (BCT one summer, AIT the next) if that works better. Once you are MOS qualified, you are exempt from the first two years of ROTC, so you can join in your Junior year as an SMP, go to Advanced Camp and commission at the end of you Senior year. If you are looking at Active Duty, you can still compete with the rest of your class as long as you are not on a Guaranteed Reserve Force Duty Scholarship. As an SMP, you will get paid at E-5 for Drill dates and AT's, get some experience beyond your peers and you will commission as a 1LT over 3 years anyway.
I recommend the Guard doing this route because most states offer a range of addition educational benefits.