Whether it’s the employer who needs to raise an issue with an employee, or an employee who would like to make a complaint to their employer, it is useful to consider in the first instance whether an informal approach could be taken to resolve the matter.
Depending on the issue it could be a simple face to face conversation. The problem is discussed, both parties gain an understanding of the cause and impact of the issue and can come to an agreement about how to resolve it and move forward. In this case it may not be necessary to have a lot of paperwork documenting the process, but it will still be useful to make a note, e.g. in a diary, that the conversation took place and a note of the outcome.
Some issues or complaints in the workplace will be of a serious nature and an informal conversation will not be appropriate or suitable to resolve it.