Start with the most recent provider
The most counterintuitive thing about hunting old medical records is how much of the work the current provider has already done for you. When a new patient walks into a primary-care office, the practice typically asks the prior provider to forward records, not always all of them, not always promptly, but often the most important pieces. The record you're looking for may already be sitting in a chart at a doctor's office two cities away from where the original event happened. The retention-rules companion explains the legal landscape; this step is about where the records tend to actually live in practice.
Ask the current primary-care doctor for a copy of the patient's complete chart, and pay attention to the word 'complete.' Not the after-visit summary. Not the printable health summary. The full designated record set under HIPAA, including the referral letters and prior-record forwards the office holds from earlier providers. Phrase the request in writing if you can; a written request triggers HIPAA's thirty-day clock and creates a paper trail you can point to if the office tells you it'll take a few months. We come back to the right language in Step 2.
A surprising amount of history surfaces from this single ask. Specialists send consultation letters back to the referring primary-care doctor by default. That's a record of what the specialist saw, often with their plan attached. Hospitals send discharge summaries to the patient's primary-care doctor as part of the standard transitions-of-care workflow. Imaging centers send formal reports to the ordering physician. The primary-care chart is therefore a thicker historical record than most patients realize; even visits to clinicians the patient has long since stopped seeing tend to leave a trace there.
If the current practice has been the patient's home for many years, this step alone may produce most of what you need. If the patient has switched primary-care doctors recently, repeat the same ask with the previous primary-care office. They almost certainly had records that the new office never asked for. Pediatric records often work this way too: the current pediatric or family-medicine practice usually has at least a summary of immunizations and major illnesses forwarded from the previous practice.