This is a very basic example of an ERC6900 compatible plugin called CounterPlugin, built in Foundry. It has one function that can be called through a user operation, called increment. In /src you will find this plugin, documented so you can understand how it works in detail.
You will also find a basic test in /test which will show this counter plugin working. Here you'll see how to setup the modular account, install the plugins and send a user operation specifying the intent to increment the count. Use forge test to run these tests.
Feel free to modify the plugin and tests to challenge your understanding of ERC6900 plugins, or use this to start building your own plugin!