Step 1: Cut a rectangle the size you think will work for you.
Step 4: Make small pleats (start and stop the pleat so the paper is folded down)
Step 5: Cut a small cut in the middle of the pleated area (I cut mine about 1/2" up from the bottom) and fold one side up
Step 6: Fold up other side
Step 7: Fan out at top and attach to layout or card. This is where you can tie it, glue it or do what you want to make it look like a shell.
See my completed layout using this technique.