A very detailed steps are provided by @paper13579. User latest nicki root version given by op.
First you have to flash 4.2 jb and then fully root using cydia impactor. And then follow the steps given by op or in page 9 of this thread.
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Using Xperia M Dual C2004
Android 4.3(15.5.A.0.18)/Rooted with Locked Bootloader