FIRST - BACKUP YOUR DATA.
Once you've done that, shut down your DNS-323, install the second drive, which must be as large as, or larger, than the first, and must be empty (new is preferable), power the DNS-323 back on and log in to the admin web page.
You should get a screen telling you the system has a new drive and prompting you to format it, there will be a check box to create a RAID1 array, check the box and follow through with the various prompts for OK/NEXT, and the unit will format the drive and then sync the data.
One of the screens will tell you that the data on the existing drive will not be lost - generally speaking this is correct, but occasionally the system will glitch and you may lose the data, hence the need to back it up, just in case.