I have just been informed that my keys are ready for collection for my second HDB flat. What are the next steps since I have not sold off my existing flat?

You will first be granted a loan at commercial interest rates. Once you have sold your current HDB flat, you must use the CPF refund and some of the cash proceeds from that sale to repay your new housing loan. After that, the interest rate on your second loan will be converted to the concessionary rate.