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straight.plugin is very easy.
Loading all the classes inside a namespace which are subclasses of some base is the preferred invocation of straight.plugin loading, and is straight forward.
from straight.plugin import load
plugins = load('myplugins', subclasses=FileHandler)
Plugins are found from a namespace, which means the above example will look for plugins in modules you could access by importing myplugins.this or myplugins.that, where the modules exist under the myplugins package. The distinction between a normal package and a namespace package is very important here, because a namespace package can add modules and other packages into an existing package, but exists in a separate location on disk.
The advantage of this is how easily you can create a Python application or library which others can easily extend. By documenting the plugins your project loads internally, you can allow others to distribution their own namespace packages injecting new plugin modules where your code will look for them, adding new functionality and behavior without any change on your part, and allowing users to customize their setup by what plugins they install in their envrionment. You can even split up your project, and release some components as optional additions to your core package.
The easiest way to get started is to start loading existing modules or classes with the straight.plugin loaders, which creates points where your project can be extended by injecting new modules into that namespace.
Read the Plugin Loaders documentation to get started.