Well...your description is of something very simple:
int itemIndex = 2;
List<string> words = MyMethod(listItems[itemIndex]);
So I can't help thinking that isn't what you actually want...but I can't work out what else you mean.
"How Declare "MyMethod" please guide me"
private List<string> MyMethod(string s)
{
List<string> result = new List<string>();
...
return result;
}