Conversation Extension steps

How can I create conversation extension ?

I am trying to ask custom questions at end of issue resolution, but we dont know where to add the code. 

https://developer.zendesk.com/documentation/zendesk-web-widget-sdks/conversation_extensions/?_ga=2.1234033.1937617194.1720432128-2021468168.1715704007&_gl=1*16b128j*_ga*MjAyMTQ2ODE2OC4xNzE1NzA0MDA3*_ga_0G6FC9CS2V*MTcyMDQzNTYxNy4yMi4xLjE3MjA0MzU2ODUuNTkuMC4w