diff --git a/non-ros-files/instructions_custom_topics.md b/non-ros-files/instructions_custom_topics.md
index e041c60..ab50378 100644
--- a/non-ros-files/instructions_custom_topics.md
+++ b/non-ros-files/instructions_custom_topics.md
@@ -163,17 +163,38 @@ exchanging _package_name_ with the source package of the custom message type (he
    <exec_depend>pubsub_msg</exec_depend>
 ```
 
-### 4. Build Package
+### 4. Configure setup.py
+The file setup.py is used to create entry points, meaning you link your python scripts to names which you can be called through `ros2 run ...`. In order for this to work your python scripts must be written using the following mechanism
+```python
+def main():
+  # main code
+
+if __name__ == "__main__":
+  main()
+```
+in which you put all of your code inside `main()`. This prevents any code from being run, if this script is not run as *"__main__"*. 
+Linking your scripts to names is done inside the file _setup.py_ by defining entry points:
+```python
+entry_points={
+   'console_scripts': [
+       'talker = pubsub.talker:main',
+       'listener = pubsub.listener:main'
+   ],
+},
+```
+The _setup.py_-excerp above links the the function `main()` from the scripts _talker.py_ and _listener.py_ from inside `/pubsub` to the names _talker_ and _listener_. This way they can be called using `ros2 run pubsub talker` or `ros2 run pubsub listener`.
+
+### 5. Build Package
 Now you can build the Python package.  
 * Move to your workspace's root: `cd ~/<workspace_path>`
 * Build workspace: `colcon build --symlink-install`
 
-### 5. Source newly built workspace
+### 6. Source newly built workspace
 * Run: `source ~/<workspace_path>/install/local_setup.bash`
 * If you already [updated your .bashrc file](https://github.com/patrickw135/pubsub/blob/master/bashrc_addons.txt) you can close all open consoles and start a new console (Ctrl+Alt+T). This will source your workspace automatically, as .bashrc is run every time you start a console.  
 __Important__: If you use multiple workspaces make sure you have the wanted workspace defined in .bashrc! Otherwise the changes introduced when building will not be available.
 
-### 6. Run scripts
+### 7. Run scripts
 * Run Talker: `ros2 run pubsub talker`
 * Run Listener: `ros2 run pubsub listener`