Click here to enlarge
Before you downgrade, please remember you can only downgrade your iDevice firmware if, and only if, you have saved your SHSH blobs somewhere safely, hopefully with Saurik’s Cydia server, or locally on your machine.
From now on, I will assume you have saved your blobs on Saurik’s Cydia server.

STEP 1: To start with, you need to make sure you have a copy of iOS 4.3.5 or whatever firmware you need to downgrade for your device which you can find and download from here.
STEP 2: Next, you need to make sure your hosts file is pointing to Saurik’s server, and not Apple’s. If you haven’t done this then please check this guide.

Alternatively, use TinyUmbrella to make iTunes look at Cydia’s Server in the Advanced options.

Click here to enlarge

Now plug in your iOS device, and switch it off, but leave it plugged in.

STEP 4: Put your device into DFU mode, our guide here.

STEP 5: Select your firmware manually to restore to. (Shift on Windows, and option on a Mac)
Click here to enlarge

It says that it will talk to Apple when really, it is talking to the Cydia servers.

Click here to enlargeHey presto! Your device should now have downgraded without a hitch…
But, should your phone should now have an error, such as error 1015, DO NOT PANIC! This is normal and has a quick fix. Just download anduse Recboot to kick it out of recovery mode –
Click here to enlarge