How To Downgrade iOS 4.3 to iOS 4.2.1, iOS 4.1 on iPhone, iPod and iPad

If you have upgraded your iPad, iPhone 4, iPhone 3GS or iPod Touch 4G/3G to iOS 4.3 for testing purpose and now you are facing any issues with your favourite apps on this new release or you want to downgrade your device to jailbreakable iOS 4.2.1 / iOS 4.1, here is complete step by step guide to downgrade iOS 4.3 to previous iOS versions.