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.