Samples for determining bulk density must be collected very carefully to ensure the sample represents the in situ condition desired and no additional compaction or loosening has occurred. One method for determining bulk density is the “core” method. A (relatively) undisturbed, cylindrical soil core (of known volume) is collected using a device like the one in Figure 2.
The driving weight is raised and dropped repeatedly to drive the sampler into the soil. When the desired depth is reached, the device is removed from the soil, and the removable metal cylinder containing the soil sample is removed. With the dimensions of the cylinder and the weight of oven-dry soil inside the cylinder, we can calculate the bulk density.
Please watch the following video for proper bulk density soil sampling technique; the "core method" is what you will be using, which begins at 22:20.
Video: A video job aid on the three preferred methods for collecting bulk density samples in soils using National Cooperative Soil Survey methods and references. https://youtu.be/E7BSZrJ-TDw?t=1341