N/V

There are two ways to practice Katakana letters on this app.
If you tap "Letter list" button, what you would see is a list of Katakana letters. As you skim the list, you would see how to pronounce the letter. If you tap one of those Katakana letters, you are taken to the next page where you can practice how to write its Katakana letter.
On the navigation bar that appears at the top edge of the Apple Watch, how to pronounce is shown in the center, and "Retry" button with a counterclockwise arrow is shown on the right side. At the bottom of the screen, the gauge with numbers is shown, which tells you how far you have written. By the way, this background pattern is called Seigaiha, which is known one of traditional Japanese patterns.
After you finish writing the letter, a semitransparent sheet of "You are done!" is shown on which you can also make sure Roman letters notation.

Next, if you tap "Word list" button, you are taken to the Group list page, such as "[ア] Group: list", "[カ] Group: list", "[サ] Group: list".
And, if you choose "[__] Group: list" button, in which there are some Katakana words list that start the letter "[__] " on the left side of the button, such as "[ア] ", "[カ] ", "[サ] ", you are taken to "__ : Word list" page that belongs to "[__] Group: list" you just choose.
Then, if you choose "__ : Word list" button, in which there are some Katakana words list that start the letter "__ :" on the left side of the button, such as "ア :", "イ :", "ウ :", you are finally taken to Katakana words list page, in which there are some Katakana words start the Katakana letter you just choose.
On the "__ : Word list" page, if you tap one of those words on the list, you are taken to the next page where you can see both English word and Japanese word written in Katakana letters.
And then, if you want to practice one of those Katakana letters, please tap one of them. After that, "Let's Practice" button turns an active state. If you tap the button, you are taken to the next page where you can practice how to write its Katakana letter.
On the navigation bar that appears at the top edge of the Apple Watch, how to pronounce is shown in the center, and "Retry" button with a counterclockwise arrow is shown on the right side. At the bottom of the screen, the gauge with numbers is shown, which tells you how far you have written.

Here is an example. 
Let’s say, if you want to say that you ate an omelet. If you only use Hiragana to write this, it would be
(A) わたし　は　おむれつ　を　たべた [watasi wa omuretu wo tabeta].
Then, if you replace some words with Kanji character, it would be
(B) 私　は　おむれつ　を　食べた [watasi wa omuretu wo tabeta].
And then, if you replace an imported word with Katakana, it would be
(C) 私　は　オムレツ　を　食べた [watasi wa omuretu wo tabeta].
Even though the face of (C) is different from the face of (A) and (B), all of them are exactly the same meaning. What is difference between (A) and (B) is that 2 words are replace to Kanji characters. And, what is difference between (B) and (C) is that 1 word is replace to Katakana characters.
- わたし or 私, means [ I ].
- おむれつ or オムレツ, means [ omelet ].
- たべた or 食べた, means [ ate ].
If you see a sentence (C), you will notice that there are still some Hiragana letters left and wonder why these letters are still written in Hiragana. The answer to your question is that these are the letters that are not replaceable to Kanji and Katakana letters.
Lastly, I would like you to feel like having a bit of Katakana letters up your Apple Watch, as the saying goes, "have something up one's sleeve."