So no need to turn ON but connect Sony ST26i to the PC you’re working on, so it will searching for driver, You can installed manually the driver through the flashtool driver located at C:/flashtool/drivers, Run and tick necessaries driver for xperia J
Copy the firmware of ST26i to the Folder c:/Users/[user_name]/.flashtool/firmwares
Still connecting ST26i to the PC, Open Flashtool, it will sync to github, just wait until sync finished, it will show device connected, click Flash Button
Bootmode chooser, choose Flashmode and OK, so the list of firmware located at c:/Users/[user_name]/.flashtool/firmwares
so select xperia JST26i firmware, it’s recommended to check Wipe: User Data (make sure backup first if got important data) and click Flash button,
wait few second for the flashtool preparing the necessaries files, once done a new dialog box will appear instructing to
disconnect ST26i from PC, turn OFf ST26i, and hold volume down button while at the same time reconnect ST26i to the PC
then if ithe necessaries flashmode driver already installed, the flashing process will continue, just wait until finished and the log report will instructed you to disconnect ST26i from PC and turn it ON to continue installing the applications and setting.