Restoring iPhone to 4.3.3 with baseband 6.15.00

Direct restore with stock 4.3.3 will throw error 1015. After hours of googling and following several tutorials, including this one

http://mobile.downloadatoz.com/tutorial/2122,how-to-fix-ios-4-3-1-iphone-3gs-bb-6-15-00-error-1015-with-sn0wbreeze-2-5-custom-firmware.html

, which after that the iPhone was unbootable.

Turns out using Pwnagetools to build custom IPSW with 4.3.3 and install with iTune will solve this problem.