That's easy. Grab HeXEn (Google Heimdall's Xbox Engineering Disc), burn to DVD-R. Put it in the DVD drive, connect the new hard drive, boot the Xbox. Once HeXEn boots (about 50 seconds) select option 3.5.3. New Disk, NTSC Xbox or option 3.5.3. New Disk, PAL Xbox - the Passcode for both is AYBX.