I recently got my Home Assistant instance setup with Alexa Smart Home and improved the process for myself by building a Cloudformation template to handle all the required AWS resources.
I made a short blog post, which includes links to launch the stack directly as well as the template. Saves some time and copy/paste when going through the Alexa setup!
https://mountainroost.com/2020/10/15/linking-alexa-with-home-assistant.html