Hi Peter,

I don’t recall a specific application-writer's-guide but the Driver Writer's Guide and the UEFI Specification are quite comprehensive.

That said, you want to have a look at existing applications, particularly at MdePkg/Application/HelloWorld to see how they are built. In particular, you will need these non-obvious things:

Your application will need to use UefiApplicationEntryPointLib to specify which function is the entry point.
Your application will need its own .inf module file and you will need to specify the .inf file in the platform .dsc file in the [Components] section. You then build your application by building that platform.

Hope that gets you started.

Cheers,
Tom