Start with the timeline
First write what happened, from evidence and without interpretation: time, place, who was where, which equipment was in what state. Then for each fact ask "why was this possible" and let the answers branch. Looking for a single root cause is wrong in most events; record separately which barriers were absent and which were present but did not work.
Two common mistakes
The first is ending the analysis at a person: "the worker was careless". That is not a finding, it is where the explanation stopped. The second is closing the corrective action with "training will be provided"; training is ineffective against a cause that was never about missing knowledge.
An analysis is not complete until the finding is tied to a corrective action and that action to a verification. In Optifora the incident record, the barrier inventory and the task list sit in the same chain.