When we performed this concept in science class, we started by estimating the volume of that object. Then, we filled a graduated cylinder with water. Then we wrote down the original volume of the water. We placed the object into the cylinder, and the volume of the water should increased (but it didn't always.) The last step was to subtract the volume of the water before the object was put in it from the the volume with the object in it. That is how we got the volume of an irregular object.