As “Delete Records” is one of the data element, it will have directly impact on your real records so use it carefully. There are two ways to define what records to delete:
1. Use a Record or a Record Collection
When you stored a record inside a record variable or a collection variable, the Id should automatically be saved in there as well. Then in the Delete Records element, you can directly assign that variable for deletion.
The most common case is to create a record variable that allows for input and enter it here, so when the flow is trigger, the system will catch the record Id, and then this record will be deleted
2. Specify Conditions
If you are overly cautious as I am, you might find this method safer. Here you can manually key in all the conditions for deletion, so you can double check that you are not deleting the wrong records.