OpenSea and other secondary marketplaces will automatically generate a collection for you as soon as NFTs are owned from your contract.
On NFT Collection and Edition contracts, your collection will appear on secondary marketplaces as soon as you mint your first NFT.
On NFT Drop, Signature Drop, and Edition drop contracts, your collection (and minted NFTs) will appear on secondary marketplaces your first NFT is claimed from your drop. Lazy-minted NFTs will not appear.
Once the first NFT is owned from your contract, you are able to edit collection information on these secondary marketplaces.
If you already have NFTs owned from your contract and your collection still isn’t showing up, see this troubleshooting guide here.