Flashing Custom ROMs for Xperia P

STEPS:
Download any Custom ROM for Xperia P. You can get a Index of ROMs, Kernels etc from here.
Do read instructions in the thread because every ROM may have a different requirement of Kernel and way of installing but basic idea is same.
Place your ROM i.e. xxx.zip file in your Root of SDCard (in internal storage)
Power off the device 
Power on and repeatedly press volume down (-) button after SONY logo appears until you see the recovery
Now to clear cache scroll down to wipe cache partition and select it and scroll down to yes
To clear data or do a factory reset scroll down to wipe data/factory reset and select it and scroll down to yes
To clear dalvik cache scroll down to advanced select it again scroll down and select wipe dalvik cache
After you perform all the steps press back to enter main menu 
Scroll down to install zip from sdcard and select it
Scroll down to choose zip from sdcard and select it
Now scroll to the ROM i.e. xxx.zip and select it and scroll down to yes to confirm.
Press back and reboot system.
Enjoy.