Thanks for stopping by the Apple Support Communities. If I understand your post, you'd like to use Scribble on your Apple Watch to reply to messages, but aren't sure how to do it. Scribble is a convenient feature that makes using Messages on the Apple Watch not only efficient, but also fun, and I'd be happy to help!
Based on what you've described, I would recommend reviewing the following information on using Scribble (including how to enable it) from Scribble on your Apple Watch - Apple Support :
With Scribble, you can write on your watch's screen, and, as you finish each word or letter, your watch turns your writing into text.
You can use Scribble whenever you need to enter text on your Apple Watch, such as when you reply to an email. Just tap Scribble, then write a word in the text box. Or enter individual letters, numbers, and punctuation. You can also turn the Digital Crown to choose predictive text.
Have a great day!