I used shells from our boiled breakfast eggs. The trick was to keep as much as possible of the shell whole - but you don't have to blow eggs as you would do for Easter decorations. I painted three shells with light green, dark green and white acrylic paint. It takes about 24 hours to dry so you cannot do it all on the same day.
You then crush the shells into ever so small bits. It is amazing how small they need to be to look natural. I had to take off too large bits and crush them into smaller ones. There are lots of things you can do with mosaic, and I wanted to make a table. I used a plastic container lid which I filled with clay. I first thought I would simply press the egg bits into the clay, but they were far too delicate. Therefore I simply glued them on. I used tweezers and toothpicks to fix the bits. I did not use any special pattern, just mixed the three colours.