How does one retire from the Individual Ready Reserve?

Are you approaching 60 and are still IRR?
You should have been moved to the Retired Reserve at 20years unless you were still active Reserve. If this is a question of how you get there, I can answer that one.
If you are IRR and have that 20 year letter, talk to your career counselor and request that you be transferred to the Retired Reserve. You DO NOT want to be DISCHARGED. Retired Reserve still has some perks, but the big reason you want Retired Reserve is that your TIS will continue to increase until you finalize your retirement.
90 days before your 60th Birthday, drop your Retirement Package. It's available online and I don't believe that HRC sends them to your last home of record. I would also check with HRC to make sure you have credit for all your points. I point this out because NGB and HRC didn't not effectively talk to each other, so make sure they have all your records.
That's about all there is to that.
Yes, both the Army Reserve and National Guard suck at getting that information to you.

https://www.usar.army.mil/IRR/
*** Oh Great Irony***
NOTE: Soldiers often miss receiving important documents that may affect retention, retirement, benefits, etc. because they have not kept their contact information current.
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Need more clarity on what you are asking. I transitioned from the IRR to the Retired Reserve at MRD with no fuss, no muss in June of 2020. Technically didn't have to even do anything, other than select whether I wanted to go into the Retired Reserve (you do) or separate (you don't).

Like any other Reservist, you will be eligible to draw my retired pay at a nominal age 60 (57 in my case, your age may vary based on post 2008 [I think] mobilizations in support of contingency operations). At that time, well, about 9 months prior, you will need to turn in the application for Retired Pay, it'll get processed in the amount of time it takes. [I think the time it takes is too long, and think you shouldn't have to "apply", especially as you permanently miss out on some of your retired pay if you don't file timely - I know at least 2 people who had that happen... not right.]

At actual age 60, you will be eligible for TriCare Retired (and later, TriCare for Life as a Medicare supplement plan). There are vision and dental benefits in there as well - I'm a bit fuzzy on the details, but I think they are earlier. I have coverage both through my civilian employer as well as the VA in a pinch, so I really haven't started to pay attention yet.

If you're asking how do you get good years towards retirement in the IRR, that's a little trickier. I graded papers for the War College. There are similar points-only opportunities floating around, but they are pretty rare. As others have said, the correspondence course path has been closed off for quite a while. This leaves being a "drilling IRR" member ( I think this is points only as well, don't quote me) , or volunteering for short tours/mobilizations, or becoming an IMA (at which point you're not TECHNICALLY in the IRR, but the IMA program).

At any time, once you have 20 good years, you can voluntarily move into the Retired Reserve, or if you fail of getting a good year, you can be moved involuntarily - subject to a one-time waiver request.

Hopefully something in that got to the questions you are looking at.