There's nothing complicated about sampling your soil for a soil test. The important thing is that the sample accurately represent the soil of the area being sampled. Here are some guides for getting the best results.
Bennett, William F.
"How to Take a Soil Sample,"
Iowa Farm Science: Vol. 11
, Article 4.
Available at: https://lib.dr.iastate.edu/farmscience/vol11/iss3/4