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
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.)
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.)
