1) Copy the downloaded zip to your sd card.
2) Make sure your device is rooted and you have ClockWorkMod recovery installed.
3) Switch off your phone.
4) Reboot into recovery by pressing home+power button+volume up simultaneously
5) Wipe data/cache and dalvik cache
6) If you want to get old back make a backup. You must have 600 mb free space
7) Select 'install zip from sd card'
8) Navigate to the downloaded file
9) Press home button
10) Scroll down to 'yes' using volume keys and press home button.
11) Wait till the flashing process is finished.
12) Reboot and Enjoy
Xperia Tizes Rom: here