I'm not aware of any specific advice on this. From a variety of accident investigations that I have been involved in my comments / suggestions would be;
Say where you were and what you were doing. Say - in the best order you can what you actually saw - not an interpretation, a commentry - just what you saw. If you did something then you'll have to weave that into your sequence of events. If you have any relevent skills mention them.
Far too many of the statements I have dealt with are full of assumptions and interpretation. That largely devalues them and makes anyone assessing them wary.
And of course most people have a position or viewpoint and that does tend to 'colour' their view of the events.
For the 'best' outcome i.e. one that aids the investigators to get to the most accurate interpretation just state what you saw. Don't worry that in many cases it is incomplete or only tells part of the story - that is nearly always the case. No one knows all the information about someone else's actions.
Anyone who gives me the complete story is almost guaranteed to making part of it up!