If you need to use action text in Rails 6, the way to follow is very simple.

Use this task in your terminal:

rails action_text:install
After you need to update the database

rails db:migrate
Restart the server and you can use action text inside your models etc.

If you need to use webpacker to serve css files you have to move actiontext.scss in your webpacker (javascript) folder and in your application.sass add:

@import 'trix/dist/trix.css'
@import 'actiontext'