You can stake Delta tokens in the Deep Farming Vault by following this guide.
On the main page click on Vaults to open the Deep Farming Vault page.
Navigate to the Vault actions section of the vault making sure that "Stake DELTA" has been selected. Enter the amount of Delta you would like to stake or click "MAX". Depending on your type of deposit you can check or uncheck the "Burn Deposit" option. Make sure to read about Burn Deposits before selecting this option. Afterwards click the "Stake" button and confirm your transaction.
Give it a few minutes until the Balances section reflects your newly staked tokens.