Contract-level metadata

You can add a contractURI method to your ERC721 or ERC1155 contract that returns a URL for the storefront-level metadata for your contract.

contract MyCollectible is ERC721 {
    function contractURI() public view returns (string memory) {
        return "https://metadata-url.com/my-metadata";
    }
}

This URL should return data of the following format:

{
  "name": "OpenSea Creatures",
  "description": "OpenSea Creatures are adorable aquatic beings primarily for demonstrating what can be done using the OpenSea platform. Adopt one today to try out all the OpenSea buying, selling, and bidding feature set.",
  "image": "external-link-url/image.png",
  "external_link": "external-link-url",
  "seller_fee_basis_points": 100, # Indicates a 1% seller fee.
  "fee_recipient": "0xA97F337c39cccE66adfeCB2BF99C1DdC54C2D721" # Where seller fees will be paid to.
}