In the 'Booking Details' section of each property, select 'Use Security Deposit' in the bottom drop-down box of this section. This will cause the boxes to change, as shown below. You can then select the amount to be held, and, if using HolidayRentPayment, when that amount should be automatically held and released.  This is automatic hold and release feature is not possible for SagePay, where the process must be done manually instead. Save at the bottom of the page when you have set this up. 

If you use HolidayRentPayment and wish to set up an automatic security deposit held against the guest's card, you need to ensure that the guests know their card details are being stored.
You should make sure, this information is part of your Terms & Conditions.

If you do not wish to automatically store card details, you would just need to contact the guest and enter new card details manually in order to take the hold.