With cameras in everything from microscopes to telescopes to satellites, scientists produce image data in countless formats, shapes, sizes, and dimensions. Python provides a rich ecosystem of libraries to make sense of them. napari is a Python library for interactive multidimensional image visualization, but it does double duty as a standalone application that can be easily extended with GUI tools for analysis, visualization, and annotation. In this tutorial, we'll start with the basics of interacting with the napari interface. Then we will show how to extend napari, from script, with your own functions and widgets. At the end, we will describe how to convert such custom modifications into a plugin that can be easily shared with other people as a Python package.