Note :: Only works for A4 devices eg - iPhone 4
Note :: This is a tethered jailbreak
1) Upgrade to iOS 6.0 / 6.0.1
2) Download the newest redsn0w
3) Download the 6.0 ipsw (the file name should be ~ iPhone3,1_6.0_10A403_Restore.ipsw)
4) open Redsn0w
5) click extra
6) select ipsw -> use the 6.0 ipsw
--if you dont use this 6.0 ipsw, redsn0w will prompt install error)
7) hit back
8) follow on screen instructions for jailbreak
9) go to extra again -> select ipsw (use the 6.0 ipsw)
10) just boot
Slightly modified compare to the reference given (its a bit incomplete)
ref: http://www.redsn0w.us/2012/11/jailbreak-ios-601-a5-iphone-and-ipod.html
if cydia does not show up
11)
Use the AutoInstall folder: If you don't have iFile on your device, you can use a desktop application such as iExplorer, DiskAid, or Phone Disk to access the device's filesystem and install Cydia. First open the application and navigate to (or create, if necessary) this directory: /var/root/Media/Cydia/AutoInstall - and then download the following file and put it in that directory: http://apt.saurik.com/debs/cydia_1.1.8_iphoneos-arm.deb. Reboot your device with the redsn0w just boot option, and Cydia should be installed.
ref: http://www.jailbreakqa.com/questions/32462/frequently-asked-questions#116177
if the repo for apt.saurik.com is empty and missing com.saurik.substrate.safemode (most tweaks depends on this)
12)
Download: http://apt.saurik.com/debs/com.saurik.substrate.safemode_0.9.3999_iphoneos-arm.deb
put it in the AutoInstall folder and reboot using the redsn0w just boot option
13) open cydia app again, the source should start downloading and substrate safemode should be installed.