I thought it would be nice to show how the process is done with the equipment that's actually designed for the process. Below you'll find the basic steps for taking a bulk density sample.
Here are the tools for the job. A steel coring tube, a plastic top for pounding the coring tube into the soil, a mallet, scissors, and a rod (not pictured) which fits into the hole you see at the top of the tube.
The first step is to remove the biomass from the soil surface where you want to take your sample. Use the scissors to remove an area slightly larger than the area of the coring tube.
Next pound the coring tube into the soil. This particular coring tube is marked out every 5 cm. Let's say we want to measure the bulk density of the soil from 0-5 cm. We'll hammer the tube into the soil until the first measurment line is level with the soil, ie when the tube is 5 cm deep.
To remove the tube from the soil, insert the metal rod into the holes at the top of the tube. Rotate the tube while pulling up. It's important to be gentle while removing the tube lest the soil sample inside come lose and fall out the bottom of the tube.
Dump the soil from the coring tube into a plastic bag. Allow the sample to dry completely. Weigh the soil, then divide this weight by the volume of the 0-5 cm portion of the coring tube. This is your bulk density.
If you want to then sample the density of the soil at 5-10 cm, put the coring tube back into the hole you have made, being careful that the sides of the hole do not cave in. The coring tube should sit with the soil level with the 5 cm mark. Pound the tube into the ground so that the 10 cm mark is level with the soil. Then repeat the process of removing the tube and putting the sample into a plastic bag. This will be bulk density of soil from 5-10cm deep. On this particular tube you can continue sampling in 5 cm increments until 30 cm deep. Other coring tubes will allow you to sample deeper depths. For greater accuracy, an augured soil corer can be used.