To add non-diegetic music to a film, first search for the desired song on YouTube and copy its URL. Paste the URL into a YouTube converter to download the song as an MP3 file. Save the MP3 and drag it into the film editing software, such as Premier, placing it in the audio section where wanted. Use the razor blade tool to cut out the desired part of the song to create a specific effect, such as creating a gloomy atmosphere.