They are super easy! I made these for my cousins but I have a ton of supplies left over so I've thought about selling them on Etsy. We'll see. I bought my chains from the Army Navy Store online and the charms came from Michaels. I think what I like about these the most is that it is a simple necklace that you can wear with a TShirt.
Here is the link I used to make my washer necklaces:
Washer necklace tutorial