In the diary you can record daily which drug you use (NPS, cannabis, alcohol, ..) and at what hour of the day. In this way you get a good overview of your use: at what times of day, during which days of the week or periods of the month.
Keeping the diary up can be confrontational, but it will help you to gain insight into yourself. Keeping a diary can play a role in getting more self-control.
It is a useful tool to gain insight into your use, to get more grip and maybe with stopping or decreasing.