Your mod loader entrypoints will reference your mod's main class. This class usually just has a static initialize(BalmRegistrars registrars) method that you can use to register all content in your mod and define any event handlers you need.
public class Waystones {
public static final String MOD_ID = "waystones";
public static void initialize(BalmRegistrars registrars) {
// Register your content using the methods on the registrars instance
// e.g. registrars.blocks(ModBlocks::initialize);
// Register event handlers for any of Balm's common callbacks
ServerPlayerCallback.Join.EVENT.register(player -> {
System.out.println("Player " + player + " has just joined the game!");
});
}
}