i am so glad you asked this magi! i love to talk and learn about old traditional methods of divination.
while intuition is really important in all methods of divination, i believe there is much to be learned from the old ways too.
the back side of the cowrie is called the 'eye'.
the side with the opening is called the 'mouth'.
the size of the shells have meaning too. the larger shells are considered 'males', smaller are 'females'.
i assume even the colorations, and marking variations have traditional meanings, but i am not familiar with any of those.
if you are casting with two shells, and they both show the eye, this is said to be good luck. if you were asking a yes/no question, this means 'yes'.
if both show the mouth, there could be problems. if you were asking a yes/no question, this would mean no.
if one side shows the mouth and one the eye, things are in the process of changing. you may need to ask again later.
as vilyariel mentioned, casting may also be done on a marked off surface, such as a bowl, drum or cloth, with the areas the shells fall in having specific meanings.