Once downloaded, this skin is automatically added to your skin collection. To apply it, go to Settings > Language&keyboard > Xperia Japanese keyboard > Select keyboard skin. You can also jump to Xperia Japanese keyboard setting by longpressing the mode key (note1) to display the pop-up Window and tapping the settings icon (note2)
(note1)
Bottom left key for switching between Kana mode and Alphabet mode
(note2)
Spanner icon