Is this a witness statement involving an aircraft ??
Witness statements should be taken by an appropriate authority ie the police or maybe even the AAIB .
They are generally required to present written evidence which may later be required in court.
They should be signed and dated and the signature should confirm an understanding of the legislation laid down in the relevant acts (criminal justice and magistrate courts act)
It is very rare , and in fact I have never heard of anyone completing one unaided with the exception of people like store detectives in the case of shoplifters .
They are normally taken by the authority and written for you .you then read, sign and agree correct .
There are many rules regarding evidence and admissibility of evidence and most of this will be missed if the author is not trained to be a statement taker.
Ask specifically exactly what the requestee wants from you and how they wish to go about it .Any probs pm me and I will try and help.
If you wish to look at a police witness statement form have a google search looking for police form MG11 there will e an example on here somewhere im sure.