Each MAC has a FOIA process page. CMS also has a rather gaudy consumer-facing FOIA page, here.
What I had never noticed, CMS has a "FOIA Reading Room" with some interesting documents. It's here. There is a 26-page PDF guide to the CMS FOIA process, here. Requests can be escalated higher in the agency, if denied.
From the opposite perspective, avoiding information release under FOIA, CMS's 26-page PDF discusses exemptions from release (primarily FOIA exemption #4, trade secrets or confidential information. Stamp your documents accordingly.) However, DOJ has a webpage stating that courts expect FOIA exemptions from release (under 5 USC 552(b)) to be narrow, not wide (here). National Parks v Morton, 1974, et seq. You can't gratuitously claim that everything you send to CMS is exempt confidential information.
What caught my attention this morning, CMS maintains public line item FOIA logs monthly back to 2015. For example, the most recent is a FOIA log for June 2018 (here). It looks like they get over 150 requests a month. Requests come from law firms large and small, newspapers, research groups, and other entities as diverse as the health plan Oscar, the Democratic National Committee, Buzzfeed, and Providence St Joseph health system. In addition to the FOIA spreadsheet reports, the 26-page CMS PDF notes that "CMS receives the highest number of FOIA requests of any agency...in FY2011, CMS received over 52,116 FOIA requests."
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I've always heard that FOIA allows requests for existing documents (e.g. all emails with keyword X from person A to person B) but never requires the agency to undertake creation of new documents.
FOIA exemption 2 exempts "internal personnel rules and practices" from release. I used to hear that this allowed an agency to refuse to release its own internal "deliberative" documents. However, the CMS 26 page PDF (here) on page 10 informs us that based on a 2011 Supreme Court case, this exemption-from-release has now been read to apply solely and literally to personnel rules. Thus, it now is understood as exempting release of [personnel rules and personnel practices] and no longer read as broadly allowing the agency to withhold [personnel rules and (internal agency) practices (of all types).]
FOIA responses aren't necessarily internally consistent. I once asked MolDx under FOIA for its scope of work (statement of work), and received a reply this was a confidential internal agency document. However, the very same request to CMS got me a copy of the MolDx statement of work in a couple weeks (here).