Hello Barry. First and foremost, information labelling is a control. Controls are implemented when there is a need for it as identified by your risk assessment process.
In the event that you need to implement information labelling on electronic documents, you can do so several ways:
1. Creating a standard template, where the footer or the header contains the information classification.
2. Putting a watermark on the page.
3. For emails, a disclaimer under the signature portion is normally done.
I hope I have answered your queries.