Use iFaith (v1.5.7) and download your shsh blobs from cydia's server and see if it says your blobs are valid or not
edit:
Download the 6.0.1/6.1/6.1.2 shsh blob for your device using iFaith. (you can only continue if ifaith deems them valid, or else your only option is 5.1.1 if you have blobs for that)
Open iFaith and click created sign ipsw with blob. Select the shsh blob you just download. It'll verify the blob, then click "download for me". After it is done, choose the ipsw if it asks you to and then click continue. It'll stitch the blob to the firmware. Once that's done, follow the instructions to put your device in pwned DFU mode
Open iTunes, shift + click restore and then choose that custom firmware
Once it's on 6.0.1/6.1/6.1.2, download evasi0n from
evasi0n.com and untether jailbreak your device
Good luck!