1) Make sure you Iphone is detected.
2) check "Advanced Options."
3) In new window, under "Device / Version," select the firmware version you want to downgrade to.
4) Click on "Save my SHSH." Pray Cydia has them. If you don't get an error, it has been saved onto your computer.
5) Repeat to save other firmware hashes if you wish.
6) Now, click on "Display SHSHs." It should listed all hashes you have saved on your computer and the firmware you can restore to.
7) Start "TSS server," download firmware file somewhere (google) and restore through iTunes with that file.
8) If you phone get stuck during restore, try "Kick out of recovery..."'in Tinyumbrella.