Step-by-step restore iOS5.0.1 WITH SHSH

Had just restored my iPhone 4 GSM to iOS5.0.1 two days ago. Was confusing what to do to restore to iOS 5.0.1 which Apple had stopped signing (and they had implemented new APTicket method for restoring process), luckily I found this FAQ and it provides complete step-by-step guide to restore. Just to share here in case anyone needs it.
Source:  http://www.jailbreakqa.com/questions/32462/frequently-asked-questions#94647

How do I restore (downgrade/upgrade) my device to iOS 5.0.1 or other non-latest iOS versions?

Since Apple has introduced an APTicket/APNonce check for restores to iOS 5, the TinyUmbrella TSS Server procedure does not work anymore when restoring to any version of iOS 5.x (you can still use that procedure for restoring to iOS 4.x if you want, but the following stitching procedure should work too). Instead, you need to create and restore to a custom IPSW (custom firmware file) with stitched SHSH blobs. (If you're lucky enough to have an old-bootrom iPhone 3GS, you don't need SHSH blobs and can go through this process without doing the stitching steps.)
To successfully restore to iOS 5.0.1, you must have iOS 5.0.1 SHSH blobs saved for this device in your computer (what are SHSH blobs and why are they necessary?). If your device was previously jailbroken, Cydia may have been able to automatically save iOS 5.0.1 SHSH blobs for your device. You can try retrieving SHSH from Cydia's servers by running TinyUmbrella, plugging your device into your computer, clicking the "Advanced" tab, checking "Request SHSH from Cydia", noting where the SHSH will be saved in "Save SHSH Folder", unchecking "Set Host File to Cydia on Exit", and then clicking "Save SHSH" at top right. Verify that your 5.0.1 SHSH blobs are saved (in the General tab) and close TinyUmbrella before proceeding. (If you're interested in restoring to a non-latest version of iOS other than 5.0.1, mentally replace 5.0.1 with your desired iOS version number and follow the same instructions.)

Instructions for iPhone 4S and iPad 2

Follow these steps to use SHSH blobs to restore your device to iOS 5.0.1 using redsn0w. If you have an iPhone, note that this method will upgrade your device's baseband to the latest baseband, which you probably do not want if you use a baseband-dependent unofficial carrier unlock.

• Grab the IPSW files (firmware files) for iOS 5.1.1 and 5.0.1 for your device - you can look for them on your computer or download them via these links.
• Open the latest version of redsn0w, click "Extras", click "Even more", and click "Restore".
• Click "IPSW", select your iOS 5.0.1 file, and then select your iOS 5.1.1 file.
• If you've already saved iOS 5.0.1 SHSH blobs to your computer with TinyUmbrella as described above, click "Local" and select your SHSH blobs. If your iOS 5.0.1 SHSH blobs are still saved on Cydia's servers, click "Remote" to grab them.
• redsn0w will install iOS 5.1.1 on your device and then downgrade it to iOS 5.0.1.

If your device gets stuck in "Connect to iTunes" mode, try opening redsn0w, clicking "Extras", and clicking "Recovery fix".

Instructions for iPhone 4 and 3GS, iPad 1, and iPod touch 3rd and 4th generation

After you have 5.0.1 SHSH blobs saved on your computer (as described above), follow these steps to stitch a "Custom IPSW with blobs" using redsn0w (compatible with Mac and Windows) and then restore with it via iTunes. If you have an iPhone, this method will preserve your device's baseband.

• Open the recommended current version of redsn0w: redsn0w 0.9.10b8b, available from the Dev Team Blog.
• If your device is an iPhone, first create a NO_BB ("no baseband") custom IPSW using redsn0w and then follow the rest of these steps. (To do this, open redsn0w, click "Extras", and click "Custom IPSW".) If your device is an iPad or iPod touch, ignore this step.
• Click "Extras".
• Click "SHSH blobs" and then click "Stitch".
• Locate the IPSW for the version of iOS you want to downgrade or upgrade to. If your device is an iPhone, select the NO_BB custom IPSW you made earlier. (You can also make a custom IPSW using sn0wbreeze if you prefer; see below for links.) If your device is an iPad or iPod touch, you can use a stock IPSW from Apple; this may be already on your computer.
• Click "Local" and select the saved 5.0.1 SHSH blob you saved in your computer earlier.
• redsn0w will now begin creating your signed IPSW. It will be saved in the same location the stock IPSW was located.
• Enter Pwned DFU mode with redsn0w or iREB. (In redsn0w, go back to the Extras menu and click "Pwned DFU".)
• Open iTunes and hold down the "Shift" key (on Windows) or "Option" key (on Mac) and click on Restore.
• Navigate to your iOS 5.0.1 signed custom IPSW and select it.
• The rest will be a normal restore process. You should now have successfully restored your device to iOS 5.0.1.

If you use Windows, an alternative to redsn0w is to use sn0wbreeze 2.9.3 in iFaith mode to create a custom IPSW with stitched blobs (which can also be useful for people who need to preserve an unlockable baseband since sn0wbreeze custom IPSWs always avoid baseband updates). sn0wbreeze is available to download here, along with the stand-alone tool iFaith that can stitch blobs: http://ih8sn0w.com/
You may also pull partial 5.0.1 SHSH blobs from your device (iPhone 4 or earlier) if it is currently on 5.0.1, using redsn0w's "Fetch" function or using iFaith.
If you get any iTunes errors while restoring, check this list of error solutions for some ideas on what might have gone wrong. If you're using the redsn0w process and you get iTunes error 1, try going through the process again but make sure to include the step of making a custom firmware that preserves your device's baseband - or try using sn0wbreeze/iFaith instead, which will also preserve your device's baseband.