I'm starting with Royale with the intention of migrating a large Flex
application to Royale. But I'm not sure yet about how to start.
Let's say I have a simple Flex test project (a Hello World one) . From
here, I want to make it a Royale one, what steps should I follow? I know how
to make a Royale Hello World but what I'm looking for is steps for
conversion a flex one.
Your Hello World example can be compared to the existing hello world in our repository . Take a look there and see much more examples. You can open almost all of them in VSCode  or Moonshine.