diff --git a/README.md b/README.md
index 5377fd5..3b7bd33 100644
--- a/README.md
+++ b/README.md
@@ -1,18 +1,24 @@
 # phlex-sinatra
 
-[Phlex](https://github.com/phlex-ruby/phlex) already works with Sinatra (and everything else) but its normal usage leaves you without access to Sinatra's standard helper methods. This integration lets you use the `url()` helper method from within a Phlex view (along with the rest of the helper methods available in a Sinatra action).
+[Phlex](https://github.com/phlex-ruby/phlex) already works with Sinatra (and everything else) but its normal usage leaves you without access to Sinatra's standard helper methods. This extension lets you use Sinatra's `url()` helper method from within a Phlex view (along with the rest of the helper methods available in a Sinatra action).
 
 ## Installation
 
-Add phlex-sinatra to your application's Gemfile and run `bundle install`.
+Add phlex-sinatra to your application's `Gemfile` and run `bundle install`.
 
 ```ruby
 gem 'phlex-sinatra'
 ```
 
+Then require it in your app:
+
+```ruby
+require 'phlex-sinatra'
+```
+
 ## Usage
 
-To enable the integration use the `phlex` method in your Sinatra action and pass an _instance_ of the Phlex view (instead of using `.call` to get its output):
+Use the `phlex` helper method to render a Phlex view and pass an _instance_ of the Phlex view (instead of using `.call` to get its output). For a classic-style Sinatra app it'll Just Work™:
 
 ```ruby
 get '/foo' do
@@ -20,6 +26,18 @@ get '/foo' do
 end
 ```
 
+For a modular app the extension must be explicitly registered:
+
+```ruby
+class MyApp < Sinatra::Base
+  helpers Phlex::Sinatra
+
+  get '/foo' do
+    phlex MyView.new
+  end
+end
+```
+
 You can now use Sinatra's `url()` helper method directly and its other methods (`params`, `request`, etc) via the `helpers` proxy:
 
 ```ruby
diff --git a/lib/phlex-sinatra.rb b/lib/phlex-sinatra.rb
index 08db3bc..128451a 100644
--- a/lib/phlex-sinatra.rb
+++ b/lib/phlex-sinatra.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'phlex'
+require 'sinatra/base'
 require_relative 'phlex/sinatra/version'
 
 module Phlex
@@ -18,26 +19,16 @@ def initialize(obj)
       end
     end
 
-    module SGML
-      module Overrides
-        def helpers
-          @_view_context
-        end
+    module SGMLOverrides
+      def helpers
+        @_view_context
+      end
 
-        def url(...)
-          helpers.url(...)
-        end
+      def url(...)
+        helpers.url(...)
       end
     end
-  end
 
-  class SGML
-    include Sinatra::SGML::Overrides
-  end
-end
-
-module Sinatra
-  module Templates
     def phlex(
       obj,
       content_type: nil,
@@ -45,13 +36,13 @@ def phlex(
       layout_engine: :erb,
       stream: false
     )
-      raise Phlex::Sinatra::TypeError.new(obj) unless obj.is_a?(Phlex::SGML)
+      raise TypeError.new(obj) unless obj.is_a?(SGML)
 
       if layout && stream
-        raise Phlex::Sinatra::ArgumentError.new('streaming is not compatible with layout')
+        raise ArgumentError.new('streaming is not compatible with layout')
       end
 
-      content_type ||= :svg if obj.is_a?(Phlex::SVG) && !layout
+      content_type ||= :svg if obj.is_a?(SVG) && !layout
       self.content_type(content_type) if content_type
 
       # Copy Sinatra's behaviour and interpret layout=true as meaning "use the
@@ -73,4 +64,8 @@ def phlex(
       end
     end
   end
+
+  SGML.include Sinatra::SGMLOverrides
 end
+
+Sinatra.helpers Phlex::Sinatra
diff --git a/spec/general_spec.rb b/spec/general_spec.rb
index 96710b6..aced156 100644
--- a/spec/general_spec.rb
+++ b/spec/general_spec.rb
@@ -36,7 +36,9 @@ def view_template
   end
 end
 
-class TestApp < Sinatra::Application
+class TestApp < Sinatra::Base
+  helpers Phlex::Sinatra
+
   set :environment, :test
 
   get '/error' do
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 2c23c2d..415d9cd 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -2,7 +2,6 @@
 
 require 'phlex-sinatra'
 require 'rack/test'
-require 'sinatra/base'
 
 RSpec.configure do |config|
   # Enable flags like --only-failures and --next-failure
diff --git a/spec/streaming_spec.rb b/spec/streaming_spec.rb
index 3d4070c..392b074 100644
--- a/spec/streaming_spec.rb
+++ b/spec/streaming_spec.rb
@@ -15,7 +15,9 @@ def view_template
   end
 end
 
-class StreamingApp < Sinatra::Application
+class StreamingApp < Sinatra::Base
+  helpers Phlex::Sinatra
+
   set :environment, :test
 
   get '/stream' do