A note made at the time is evidence. The same note made from memory the next morning is a recollection of a memory, and everybody in the courtroom will know the difference.
The mechanics.
- Date, time, and place at the head of every entry, and the time you started writing if it is not the time of the events.
- Consecutive pages, no blank spaces left "to fill in later". A gap is an invitation to allege that something was added.
- A mistake is struck through with a single line, initialled, and the correct version written after it. Never erased, never overwritten, never a page torn out.
- Who was present, including colleagues. They will be asked, and their account will be compared with yours.
Observation and inference are different sentences. "The door frame was split near the lock, and splinters lay inside the hall" is an observation. "The door had been forced from outside" is an inference. Write the first, and where you must write the second, write it as your conclusion and give the observation it rests on. An inference recorded as an observation cannot be unpicked later, even by you.
Quote, do not summarise, what people say. Especially the first thing they say, and especially if it is unhelpful. A witness's own words are evidence; your paraphrase of them is your writing.
Assume it will be read out. Notebooks are disclosable in most systems. Anything sarcastic, dismissive or speculative in a margin becomes the sentence the cross-examination begins with, and it is read as evidence of how the investigation was conducted.
A statement is not a note. It is taken from an account already given, in the person's own words as far as the format allows, read back to them, and corrected by them before signing. Do not draft it for them and ask them to agree.
Source. National Institute of Justice, Crime Scene Investigation: A Guide for Law Enforcement (NCJ 178280), https://www.ojp.gov/pdffiles1/nij/178280.pdf ; UNODC, https://www.unodc.org/ ; and the Principles on Effective Interviewing for Investigations and Information Gathering, https://interviewingprinciples.com/
What this is not. The rules of evidence of any particular jurisdiction. Format, retention and disclosure are set by the law where you work.