Find your Farm using the search field. To open details, click anywhere on the row showing your pair. Click the Enable button and confirm your action in your wallet.
After a moment, the Enable button will change to Stake LP. Click it, and a new window will appear.
Type the amount of LP Tokens you would like to farm with into the field, or just click Max to use all of your LP Tokens, then click the Confirm button. Your wallet will ask you to confirm your action.