To understand that in order to find volume and surface area of irregular solids, we must find the basic solids (prisms, cones, pyramids, etc.) that make it up, and calculate from there.
This packet explains how to find volume and surface area of irregular solids by way of determining solids we already know that make up the irregular.
This video goes over the confusing concept of finding the volume of irregular solids.
This video, like the volume one, goes through how to find the surface area of irregularly shaped objects.