Get your tie and start folding it in a way that looks cool to you. I think it's easier if you pin the folds in place
Next, start hot gluing along the folds to hold them in place. Be sure to flip it over and glue along the back as well
As you fold, make sure the necklace will fit over your head. When you're done, glue the two ends together
Add a button or some other pendant to where the two ends meet. Viola! you're done!