The Professional Agricultural Analysis Group (PAAG) have provided Tried & Tested with various sampling guides which can be found in the library section of the website here.
This month (April 2020), the short but very handy sampling guidance for Soil Mineral Nitrogen (SMN) has been updated to provide further details.
The guidance covers the principles fo sampling, and also covers the following questions:
Why sample?
How often should samples be taken?
When should samples be taken?
How should samples be taken?
How should samples be submitted?