Go far a walk on a beach and collect the prettiest seashells that you can find. Ideally, you will have a beach close to where you live. If you are not near a beach, then you can get some shells at most hobby and craft shops. You can find beads at hobby and craft shops as well, so if you plan on using beads then pick out some pretty ones that you think will coordinate well with the shells that you plan on using.
Wash off shells thoroughly with a gentle soap and some cool water, removing any sand or dirt while being careful not to break any of them.
Arrange the shells in a desirable pattern, by colors, shapes and sizes, putting beads in between them.
Take an electric drill and, using a tiny drill bit, drill a hole through the top of each of the shells. This is where the beading cord will go through.
Take some beading cord and measure it so that it is about 30 inches in length and cut it with a pair of scissors. Secure a jewelry clasp to one end of the cord.
String the shells (and the beads if desired) onto the beading cord through the drilled holes. Put them in the pattern that you arranged previously. Continue until all of the cord is strung.
Take the other end of the beading cord and attach the other part of the jewelry clasp to it to complete the necklace.