Hey guys, I'll try to install it on my friend's J tomorrow. If it's working, I'll make a video tutorial
Later Edit: Hey guys, it worked but I couldn't record because my Xperia S was running a custom ROM and the camera wasn't working. The steps I've followed are actually exactly as the ones said in the first post of this thread.
1. Make sure you are rooted
2. run install.bat from the zip(cwp_6.1 or whatever) extracted
3. press accept or grant in the supersu/superuser prompt on phone
4. after install.bat is done, press any key in the console to close it
5. if you got any errors in install.bat, run it again, it happended for me once(when it had to get root permissions on phone, the process was actually continuing)
6. turn off the phone, run Flashtool and select the recovery ftf (it's the firmware with device ST26i and Branding "Aby uzywac CWM sflasuj to") and flash it to the phone(in flashmode)
7. After the flashing is done, unplug the phone, power it up, and press vol - when Sony logo appears
8. Use CWM for whatever you need for
9. After you are done with CWM, do not reboot, remove the battery(rebooting may cause to harm your device, because recovery.ftf contains ICS kernel which is not compatible with JB)
10. put the battery back, don't power up the phone yet
11. In flashtool, select the .31 firmware(either the official ftf or the JB31.ftf containing only the kernel) and flash the kernel only
12. After flashing is finished, unplug the phone and power it
So the idea is that if you need to access the CWM, you have to power off your phone, flash the recovery.ftf, enter cwm, and after using cwm power the phone off and flash back the original kernel(31 ftf)
And yes, it was a locked bootloader, and more than that, it's a Bootloader unlock status: NO, so if it worked for me, there's no reason it won't work for you. The reason it's not working for you is that you're not following the steps correctly. Some of the users saying this isn't working is because they didn't made sure they are rooted, so once again I tell you to check if you're rooted before doing this.