If you want to join the stock income plan, you can join it directly on Futu app. Click Trading → Securities Account → U.S. Stocks → Stock Income Plan to join directly. After joining, no operation is needed. Futu Securities will manage all matters related to stock lending.
If there is income, the daily interest is calculated for you according to this formula: number of shares lent × stock price × annualized lending rate × days /360. Let me add here that Futu will charge 50% of the income as management fee, so the interest actually earned will be halved.