The video below will show you how to set up Direct Debit / Bank Transfer payment method. Don't forget once you've done this, test it out!
Once you've set this up, you should add the same details to the Order Status of 'open' as all the orders that choose the payment method of 'Direct Debit / Bank Transfer' will land on that order status, and it can be set to send an email to the customer that tells them 'your order is open'. You can change this email to read the same as what you put in the 'payment description section' so that they have a second copy of the direct debit details. See article "Setting up an order status"