WARNING, this is easy to do, but tough to master. It will take several attempts. Be patient and do this in an area you don't mind getting dirty. The eggs tend to explode when you are making them, (nothing dangerous, but messy) and you will get plaster everywhere.
Step 1: Materials
Bag of 9" balloons-round (but really Egg Shaped) clear is best, or white or yellow, easy to see through.
Empty bottles of water-1/2 liter. Easy to crush.
Mixing cup, more than 1/2 liter.
Exacto Knife or Razor Blade
Towels, drop cloth, (did I mention this is messy?)