About using flash tool in my sony xperia m dual c2005

I have some doubts about the using of flash tool.
1. Is it necessary to unlock the boatloader before flashing my phone?
2. Is it necessary to tick the usb debugging before using flash tool?
3. When i use flash tool, after some time it shows failed in my xperia m dual c2005. I am curious to know the reason behind it..
Any body please help me to clear my doubts....

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