A basic example of a custom Rails generator with the full guide and explanation available at https://garrettdimon.com/journal/posts/creating-custom-rails-geenrators

model.rb.tt

# frozen_string_literal: true

# lib/generators/plain/templates/model.rb.tt

class <%= class_name %>
<% if active_model? -%>
  include ActiveModel::Model

<% end -%>
<% if attributes_names.any? -%>
  attr_accessor <%= attributes_names.map { |name| ":#{name}" }.join(', ') %>

<% end -%>
end

model_test.rb.tt

# frozen_string_literal: true

# lib/generators/plain/templates/model_test.rb.tt

require "test_helper"

class <%= class_name %>Test < ActiveSupport::TestCase
  setup do
    @<%= singular_name %> = <%= class_name %>.new
  end

  test "the truth" do
    assert_predicate @<%= singular_name %>, present?
  end
end

plain_generator.rb

# frozen_string_literal: true

# lib/generators/plain/plain_generator.rb

class PlainGenerator < Rails::Generators::NamedBase
  # Remember that NamedBase defines the `name` argument for us, so anything we declare here will be passed in addition
  # to the `name` argument.
  argument :attributes, type: :array, default: [], banner: "attribute attribute"
  class_option :active_model, type: :boolean, default: true

  source_root File.expand_path("templates", __dir__)

  def create_model_file
    template "model.rb", File.join("app/models", "#{file_name}.rb")
  end

  def create_model_test_file
    template "model_test.rb", File.join("test/models", "#{file_name}_test.rb")
  end

  private

  def active_model?
    options[:active_model]
  end
end

plain_generator_test.rb

# frozen_string_literal: true

# test/lib/generators/plain/plain_generator_test.rb

require "test_helper"
require "generators/plain/plain_generator"

class PlainGeneratorTest < Rails::Generators::TestCase
  tests PlainGenerator
  destination Rails.root.join("tmp/generators")
  setup :prepare_destination

  test "generates the model file" do
    run_generator ["Point"]

    assert_file "app/models/point.rb" do |content|
      assert_match(/class Point/, content)
      assert_match(/include ActiveModel::Model/, content)
    end
  end

  test "generates the model test file" do
    run_generator ["Point"]

    assert_file "test/models/point_test.rb" do |content|
      assert_match(/class PointTest < ActiveSupport::TestCase/, content)
    end
  end

  test "supports specifying attr_accessor fields for the model" do
    run_generator %w[Point x y]

    assert_file "app/models/point.rb" do |content|
      assert_match(/attr_accessor :x, :y/, content)
    end
  end

  test "supports specifying --active-model for the model" do
    run_generator %w[Point x y --active-model]

    assert_file "app/models/point.rb" do |content|
      assert_match(/include ActiveModel::Model/, content)
    end
  end

  test "supports specifying or --no-active-model for the model" do
    run_generator %w[Point x y --no-active-model]

    assert_file "app/models/point.rb" do |content|
      assert_no_match(/include ActiveModel::Model/, content)
    end
  end
end

USAGE

Description:
    Generates a plain Ruby class with the given NAME

Example:
    `bin/rails generate plain Name`

    This will create:
        app/models/name.rb
        test/models/name_test.rb

    `bin/rails generate plain Point x y`

    This will create these files and add x and y attr_accessor:
        app/models/name.rb
        test/models/name_test.rb

    `bin/rails generate plain Point x y --no-active-model`

    This will create these files, add x and y attr_accessor, but skip the ActiveModel include:
        app/models/name.rb
        test/models/name_test.rb