The answers for a psychiatrist differ from a psychologist. The scope of this article does not include healthcare providers in other specialties.

Psychiatrist

The American Board of Psychiatry and Neurology

A Forensic Psychiatry fellowship is the first step for to acquire Board-Certification in Forensic Psychiatry, granted by the American Board of Psychiatry and Neurology. Fellowship is a one-year program.  Fellowships are offered at fewer than 50 schools in the US.  Information about Fellowships is available from the American Academy of Psychiatry and the Law site (AAPL). Psychiatrists can and do practice forensic psychiatry (lower case) who are not Board-Certified in Forensic Psychiatry. A psychiatrist can serve as a subject matter expert, for example. It is my understanding Fellowship positions are available and may even go unfilled until late in the usual “application cycle.” 

The American Board of Medical Specialties

Complementary training and Board-Certification in more than one specialty can make a powerful combination if the case touches is medically complex. E.g. was behavior related to a psychiatric condition or side effect of meds for another condition?

I recommend you look at The American Board of Medical Specialties report of Board-Certifications and requirements including options for cross-specialization. Read the 2019-2020 ABMS report for more information.

Psychologists

American Psychological Association article on Forensic Psychology.

For a psychologist, dedicated training is available through a wider range of pathways. Disclaimer: This is an overview and not intended to endorse a particular program or approach to building your practice.

A psychologist can pursue postgraduate study; some psychology programs are dedicated to acquiring a PhD or PsyD in both Psychology and Forensic Psychology.

Relatively recently, the ABPP began offering Board-Certification in Forensic Psychology. At last count, fewer than 400 psychologists held this Board-Certification, a significant addition to their credentials and qualifications as an Expert Witness.  Board-Certification in Forensic Psychology from the ABPP, in my opinion, is the gold standard for credentials.

Prior to obtaining a PhD or PsyD, it is possible to pursue a Masters Degree in Forensic Psychology. John Jay College of Criminal Justice, for example, is a top-ranked school for a Master’s Degree in Forensic Psychology. Also highly ranked is the University of Denver’s Master’s Degree in Forensic Psychology. Many forensic psychologists obtain their degree online through lesser known Alliant University, because they can pursue school while holding a day job.

Talk with your colleagues! Learn how they got to their forensic practice. I recommend asking them how their own education stood up to scrutiny on the stand.

Many other sites do a better job of answering this question than me and there are dozens of dedicated Forensic Psychology programs, so I will leave it to them.