You will need the firmware file specific to the D5303. While you can create this yourself using XperiFirm, many users look for pre-built "lock remove" FTF files shared by the modding community. However, to ensure compatibility with your specific bootloader status.
USB cable + PC with Windows 7/10/11.
: Install Flashtool and the driver pack. Copy your downloaded lock_remove.ftf file into the sony d5303 lock remove ftf