Katakana

SeiOn
aiueo
kakikukeko
sashisuseso
tachitsuteto
naninuneno
hahifuheho
mamimumemo
ya-yu-yo
rarirurero
wa---wo (o)
n----
DakuOn
gagigugego
zaji (zi)zuzezo
daji (di)zu (du)dedo
babibubebo
papipupepo
YoOn
キャキュキョ
kyakyukyo
シャシュショ
shashusho
チャチュチョ
chachucho
ニャニュニョ
nyanyunyo
ヒャヒュヒョ
hyahyuhyo
ミャミュミョ
myamyumyo
リャリュリョ
ryaryuryo
YooDakuOn
ギャギュギョ
gyagyugyo
ジャジュジョ
jajujo
ビャビュビョ
byabyubyo
ピャピュピョ
pyapyupyo

Since an increasing number of foreign words, some new methods have been used in order to make the sound as close as possible to the original sound of foreign words.

Example
ファフィフェ
fafife
ヴァヴィヴェ
vavive