Practice writing

How do I do the Write it! exercises on my iphone? Specifically, how do I write kana or kanji in the box labeled "Write it in Japanese here"
 

After activating the Japanese keyboard on your iphone (which you've probably already done otherwise you can follow these instructions https://kawakawalearningstudio.com/language/japanese/type-japanese-phone-iphone-android/) you're able to type in Hiragana. If you want to use Katakana and Kanji it get`s a bit tricky: From what I understand you first have type in Hiragana and then choose from the suggestions made on top.  

I like typing in Japanese because it helps me learn and reinforce the kana.