To blow out the eggs I used the two hole method. I pierced the bottom and the top of the egg with a needle, then used a bead drill to enlarge the top whole. Then I made sure the yoke was broken and covered the holes with my fingers and gave the egg a couple of shakes. After that you blow into the bottom hole and the insides come out the top hole. Fill the shell with soap water a couple time and rinse. Then I baked the egg at 300 degrees to dry it and make the shell a lil harder. Finally, I filled it with plaster of paris. For the next batch I'm going to try and fill them using a glue gun. Should end up much lighter.
Anyway, that's only one method. There's tons more online.