If there is a lot of evidence relevant to the investigation then it is a good idea to collate a list of the evidence and put it all into a folder to make it easy for you and the parties involved to review the evidence.
After each interview forward the record, as appropriate, for signing, to the interviewee. If as a result of the grievance investigation and process you find that there are recommendations for changes that the employer or HR should make then it would be good practice to put these into a separate document which you provide to the relevant person or department with a copy of the grievance report so that they can be actioned.
If you are unsure about the process or the legalities of the grievance investigation then you can seek advice but it should be limited to the process and not the content of the grievance report. It is important to remember to take a balanced approach to the investigation which means not only looking for evidence that would go against the grievance but also evidence that would support it.
Where shared, if the interviewee disagrees with an aspect of the record they will be able to annotate the notes before returning to the investigator. For example, if you are going to investigate a fire accident, you must download an investigation report template from here.
Try to be balanced in your report and your conclusions so that it is clear that you have taken a neutral position when investigating the grievance. Unlike criminal proceedings in which the standard of proof is 'beyond reasonable doubt', an investigator only needs to decide that 'on the balance of probabilities' an incident is more likely to have occurred than not.