Please make sure you have read and understood Explained: SHSH Blobs.

Method 3

Use this in the case when your device is currently running a firmware that Apple is no longer signing, and you don’t yet have the SHSH blob for it.

Step 1:
Download iFaith.

Step 2:
Plug in your device.

Step 3:
Launch iFaith and click on Dump SHSH Blobs.