On the popup menu at the top right of the screen there is a "New Kata" option.

I found the interface relatively straight forward to use.

Make sure your Kata is a new idea first by searching the existing Kata.

There are four important sections to fill out.

1. Your (hidden) solution.
2. The template for the solution that the user starts with.
3. The sample tests the user can see and edit.
4. The hidden tests required to solve. This is usually the sample tests, plus some random tests and any interesting edge cases.

https://www.codewars.com/topics/kata-best-practices