You have to make an account on net.vocaloid.com.
This is the page for Vocaloducer. Basically what you do is pick a Vocaloid (in this case Gacha, but usually it's just the VYloids for free accounts),
decide the mood (bright/neutral (they call it standard)/sad) and genre, and enter the lyrics into text boxes. I know it accepts kana and kanji, but I think romaji didn't work when I tried. Then you pick a title and tempo, hit the pink button, and it generates a song in maybe a little over a minute.