How do you measure specific gravity of sand?

Determine the density of the sand. Determine the mass of the sand, and it's volume. Divide the mass by the volume and that gives you density. Then divide the sand's density by the density of water. That will give you the specific gravity of the sand. Because you divide densities, the units cancel out, and specific gravity does not have any units.

For example, you determine the density of the sand to be 10g/cm3, and the density of pure water is known to be 1g/cm3. Divide 10g/cm3 by 1g/cm3. The g/cm3 cancel, and you are left with just the number 10. So in this example the specific gravity of sand is 10.