To download the assets from the GitHub repository:

1. Go to the following web address using your preferred browser: https://github.com/Unity-Technologies/NavMeshComponents.
2. Go to the releases section, located under the Code tab as shown:

3. Download the compressed file to your computer, according to your version of Unity. In our case, it's 2018.1.0f2 at the time of writing.

4. Decompress the file outside the main project. We can see that it is a Unity project on its own:

5. Navigate to the Assets folder, where we will see the following:

6. Copy or move the NavMeshComponents folder, with all its content, into your target project's Asset folder:

7. Let Unity reload the assets.