Without an IME, type romaji and it becomes kana as you go: ka → か, shi or si → し, kyo → きょ. Double the consonant for a small っ (kko → っこ). For ん type nn, or a single n before another consonant. Long-vowel mark ー is -, and 。 is ..
In the Kanji mode you type the reading of the word shown. With hints off, the kana are hidden until you type them, so it doubles as a reading quiz.
A wrong key shakes the line and clears what you've typed for that kana. If you use your own Japanese input method, tick the box above and just type the text, then press Enter.