At this point, the extension can be developed incrementally. If the --watch option is not being used, then at various stages, you may want to redeploy your extension to JupyterLab to check it's progress. To do this, use the following command with jlpm:

jlpm run build

At this point, the extension can be anything. Tutorials can be found here:

• Certificate mime renderer: https://github.com/jupyterlab/jupyterlab-mp4/blob/master/tutorial.md
• mp4 player: https://github.com/jupyterlab/jupyterlab-mp4/blob/master/tutorial.md
• Astronomy Photo: https://jupyterlab.readthedocs.io/en/stable/developer/extension_tutorial.html