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?)
Step 2: Mix your plaster
This is important; too thin and the walls of the egg will be too thin, too thick and it won't spread around enough. Too much water and it will take a very long time to harden. All I can say is try and try again and know it won't work the first time.
In this picture I use a piece of 1/2" PVC pipe. Not a good choice, next time I used a screwdriver to mix. Try to remove most of the lumps. I bet a paint mixer on a drill would work well.
Step 3: Pour the mix into the plastic water bottle.
A funnel will not work to get the mix into the balloon, but it would work to get the mix into the plastic bottle.
The bottle should be full or almost full of mix.
Very messy work.
Rinse bottle off.
Quickly go to next step as the plaster is hardening as we speak.