How is the density of a substance calculated?

1).  Take a sample of the substance.  The sample should be the largest
possible that will allow it to be be easily handled and the following steps
to be performed with it. 
(The density doesn't depend on the size of the sample, and every sample
of the same substance has the same density.  But using a larger sample
can improve the accuracy of the measurements you make, and therefore
improve the accuracy of the density you derive for the substance.)

2). Ask or measure the mass of the sample.

3). Ask or measure the volume of the sample.

4). Divide the mass by the volume. Their quotient is the density
of the substance.