It's an interesting and at times rather confusing process, this retiring from practice. I speak in my case of someone working for a health care corporation, so I didn't have to shut the practice down, just my own part of it.
As things went along, there was little in the way of spontaneous information coming my way, so I had to ask questions about my health insurance, my liability insurance, and so on. I found out quickly that everything stops at midnight on your last day. Since I'm over 65, I wasn't eligible for COBRA (except for dental care).
I already had Medicare Part A, since you must sign up for that, but struggled a bit to understand Part B. I registered on the Social Security website, but it takes a lot of fishing around to try to understand the process. I finally called the local SS office and received forms for my employer to fill out. This ensures I don't pay extra for signing up for Part B late.
One thing that working in rehab has taught me is that you need to have supplement insurance. You don't want a Medicare replacement policy, just a supplement. If you get a replacement policy, you hand over all the decision-making to these private companies. Here, our experience has been that Anthem and Humana can be a bit flaky with approvals, so I decided to go with a UnitedHealthcare supplement through AARP, which seems to behave pretty well. But you're not done yet, since there are standard Plan types that each company must offer. I chose Plan C, which has some additional bells and whistles like travel insurance. At this point you can sign up online, and there are links from the Social Security website for that.
I wasn't, and still am not, absolutely certain I won't go back and practice in some way, though an eye opener was learning that to pay for my current malpractice insurance would cost me $13,000 per year(!). There are some alternatives that might be more like $8-9,000, still a chunk of money, meaning I'd have to work that much just to break even. For now I just have a "volunteer" policy (costing $100), which says I can see patients as long as I don't charge them.
So now I have my coupon booklet for making my supplement premium payments, and Medicare tells me they will bill me for 3 months at a time. Once I sign up for Social Security, I understand they will take the premium for Part B out of that payment automatically.
The Social Security website is a pretty good one for finding out a lot of details, but still this was piecemeal work I had to do myself to fully understand what I needed to do when.
On to retirement!