the process for installing stock:
***first - please check flashtool tutorial for your device if not sure how to use it. i am not responsible for bigger problems coming out of small holes***
first try connecting phone in flash mode --> plug it in while holding vol. down. ( check it in forums for using flashmode if not sure )
if it works proceed below :
-first start flashtool
-choose your stock .ftf in flashmode (download it from device specific forums )
-when it asks for device to be plugged in, connect it ( while holding vol. down)
-when finished, disconnect and reboot.
remember first boot gonna take time.