SD Card - Installation of stock ROM to my device

Can anyone tell me how i write a ROM to an SD Card of a unrooted device - i cant access the SD Card on the device, as Android wont start up after a resetting of the device - so i am resetting the ROM.

Can it be done on my PC, OR another Android device - is there anything i need to do to the SD Card - make it bootable or format it in a particular way

Any help would be appreciated !!!!!!!!!!