Here's how to set a person's membership to be collected via direct-debit.
1.) Open the persons contact record via the contacts manager and fill in the BANK ACCOUNT DETAILS tab as illustrated here:
2.) Next, open the membership record via the membership manager and ensure that the payment type is set the payment type you have set up for DD collections, e.g. "BACS" or "Direct Debit" etc
3.) Open the Direct Debits Collection screen and click "Add Unpaid to List", the new membership will now be added to the DD collections list.