The Istanbul Protocol is the United Nations manual on the effective investigation and documentation of torture and other cruel, inhuman or degrading treatment. Most of what it says to a non-specialist is about restraint.
A medical evaluation is done by a qualified clinician. Not by an investigator, not by a member of an association, not by a volunteer with a camera. A non-clinician documents two things: what the person chooses to say, and what is visible without any examination at all.
Do not examine. Do not ask anybody to undress, to lift clothing, or to show you an injury on an intimate part of the body. Do not touch. Somebody who has been assaulted in custody has already had control of their body taken from them once.
Describe, do not diagnose. "A red-brown mark, oval, about three centimetres across, on the outer left forearm" is a description anybody may write. "A cigarette burn" is a clinical conclusion, and writing it converts a usable record into a contested one. Whether an injury is consistent with what the person describes is an assessment the Protocol reserves for a trained clinician.
Consent, then photographs, and in that order. Photographs of a person's injuries are taken only with informed consent, freely given after the person has been told where the images will go and who will see them, and only in a way that preserves dignity. Consent to be photographed is not consent to publication.
Safety comes before the record. For somebody still in custody, still in the country, or still reachable by the people who did it, a document with their name in it is a danger. Ask what they fear, record only what is necessary, store it encrypted, and do not carry it across a border without thinking about who reads devices at that border.
Absence of marks proves nothing. Many methods are chosen precisely because they leave none — suffocation, position, cold, sleep deprivation, threats, sexual humiliation. A record that notes "no visible injuries" and stops has documented the technique's success.
Deaths in custody are governed by the Minnesota Protocol: an investigation that is prompt, effective, independent, impartial and transparent, and a duty on the state to explain.
Source. OHCHR, Istanbul Protocol: Manual on the Effective Investigation and Documentation of Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment (revised), https://www.ohchr.org/sites/default/files/documents/publications/2022-06-29/Istanbul-Protocol_Rev2_EN.pdf ; OHCHR, The Minnesota Protocol on the Investigation of Potentially Unlawful Death, https://www.ohchr.org/sites/default/files/Documents/Publications/MinnesotaProtocol.pdf ; the Murad Code, https://www.muradcode.com/murad-code
What this is not. The association does not investigate, does not examine anybody, and does not certify that anybody was tortured. This exists so that a member who is present does not destroy what a competent authority would later need.