flutterwaylandelinux is finished and complete, the others are tests and might help you for your specific use case.
First run step1_build_yocto.sh which will compile yocto itself and install flutter-embedded.
Second run step2_build_embedded_flutter_app.sh which will copy over myproj flutter project, compile it, and put it back at the same location.
Lastly, you need a distro to run the build.
Check out step3_runonqemu64.sh for an example.