cd plugins/graphql-java-codegen-gradle-plugin/graphql-codegen-gradle-plugin
./gradlew clean install

These instructions are unfortunately not possible to follow.

1. The gradle wrapper is not present in this subfolder (which is ok, but we need to change the command to simply gradle and assume the correct version is installed globally on the machine).
2. There is no install task on the plugin project:

Task 'install' not found in project ':graphql-codegen-gradle-plugin'.

I guess it should be changed to the publishToMavenLocal task instead.

Last thing, the example project can't be built because it is commented out in settings.gradle.
However, the plugin's project can't be built the first time if the other one is uncommented, because the dependency is not declared in the build.

Maybe we can improve the projects setup so that the global build tests everything (including example projects) without having to follow a specific order and without changing directories.