To create task, you have to create class which inherits from IScheduleTask. Implement one method, Execute.
In this example we will log short message.
All tasks have to be registered.
In Install method add few lines which will add task to system.
Similarly in the method Uninstall, add code to remove task from system.
Rebuild and install plugin.
In System > Schedule tasks enable your task
Result in System > Log
In task options you can choose how often it will be runned, for example every minute.
Here you can read more about mechanism that was used to build our Tasks system.