Kettle is a library providing additional extensions and utilities for easier mod development in Kotlin.
User and Modpack Developer installation:
If you are here, you're likely attempting to install a mod that has declared Kettle as a dependency. In that case, simply install the main Kettle JAR file as a regular mod.
Mod Developer information:
Mod developers can find all information they need either at the project's repository or in the User Guide.