TinyUmbrella is used to restore firmwares that Apple is no longer signing. TinyUmbrella saves your SHSH for previous firmwares, which allows you to downgrade even when Apple is no longer signing that firmware. You can download TinyUmbrella from these locations: OSX, Windows
Now for the actual guide:
Step 1) Backup your device using iTunes.
Step 2) Download the latest TinyUmbrella from above.
Step 3) Run TinyUmbrella (On Windows, you will have to run it in Administrator Mode.) TinyUmbrella does not download your SHSH; you will have to download them from Cydia before running TinyUmbrella.
Step 4) Click "Save my SHSH" and all the SHSH on your device that has downloaded from Cydia will be copied to your computer. You should do this twice, once with the box under advanced that say "Request SHSH from Cydia" checked. and again with it unchecked.
To restore the SHSH, follow these steps:
Step 1) Press "Start TSS Server" in TinyUmbrella.
Step 2) Open iTunes and hold down Shift on Windows or Option on Mac and click on Restore for the device you would like to restore.
Step 3) A window will open in which you can chose your firmware to downgrade to.
Step 4) Now you can jailbreak the firmware that you just installed. If this device is not an iPhone 4S or iPad 2, follow the guide here. If it is, I will have a guide up shortly.