You can make a card with simply crayons and paper, or you can gather a bunch of supplies. It is entirely up to you.
You are welcome to use craft supplies, such as: stickers, stamps, etc., so long as the products are not excessive to the point of falling/rubbing off the card (this can create health concerns for some kids). This is especially important to keep in mind when using glitter. Every week, we receive cards with excessive amounts of glitter and are forced to throw those cards away.
Rubbing your finger across the glitter, stamp ink etc., to ensure that little to nothing rubs off, is typically a sufficient way to tell whether the product is excessive.