MPS-extensions is a library of MPS languages that extend core MPS functionality, such as grammar cells for writing left-to-right languages, or editor extensions for creating tables or diagram editors. It is widely used in large commercial projects based on MPS but it can be difficult to get started using it, especially if it is also the first external dependency that you are about to add to your project.
To help get you started, I have created a sample with a simple Gradle build script and an MPS build script. Find it here: https://github.com/specificlanguages/sample-using-mps-extensions.